I'm going to order them by what biome they should spawn in.  So, the biomes are rain forest, swamp, seasonal forest, forest, savanna, woods, taiga, desert, grass desert, and tundra.  
 
Group those together in their appropriate larger groups; winter biomes - taiga, tundra; savanna biomes - savanna; Plains (Australia) biomes  (was going to group this together with savanna, but I thought the animals should be kept separate) - grass desert (plains); seasonal forest biomes (aka, the normal biome) - seasonal forest; rain forest biomes - rain forest; forest biomes - forest, woods; desert biomes - desert; swamp biomes - swam
 
Winter biomes:
Penguins - passive, occasionally dance, lay eggs
http://media.scpr.org/images/2010/10/06/penguin.jpg
Arctic Hare - passive , same as bunny except different color
http://peterandthehare.files.wordp [...] ech_07.jpg
Snow Owls - passive, attacks small prey
http://www.romantic-chateau.com/im [...] barrow.jpg
Musk Oxen - neutral, drops wool, spawn in groups, milkable
http://www.alaska-in-pictures.com/ [...] n_4902.jpg
Arctic Fox - neutral, same as fox except different color
http://www.anwr.org/gallery/images/08-Arctic%20Fox.jpg
Wooly Mammoth - neutral, drops wool
http://www.prehistory.com/dinosaurs/woolym.jpg  
 Seals/Walrus - passive/aggressive respectively, near or in water, spawns in groups of seals with 1 walrus (no clubbing seals!)
http://bioweb.uwlax.edu/bio203/s20 [...] rition.jpg
http://3.bp.blogspot.com/_E2Wa1M6x [...] _large.jpg
Polar Bears - neutral, aggressive at night, attacks smaller creatures (already in Mo’ Creatures)
http://www.solcomhouse.com/images/ [...] -11-15.jpg
White Wolf - aggressive, same as wolf except different color
http://stormwolf.webs.com/Whitefur.jpg
Snow Leopards - aggressive
http://upload.wikimedia.org/wikipe [...] pard_1.jpg
 
Desert biomes:
 Camels - passive, mountable, speed of brown or black horse
http://camels.evilsun.org/camel.jpg
 Desert Fox - neutral, same as fox except different color
http://www.sinai-guide.com/Attract [...] rt-Fox.jpg
 Horned Lizards - neutral (they shoot blood from their eyes at aggressors)
http://www.wildherps.com/images/he [...] o_camo.jpg  
Scorpions - aggressive, possible poison
http://www.flightglobal.com/blogs/ [...] orpion.gif
Desert Snakes - aggressive, possibly poison, sidewinders
http://www.phschool.com/science/sc [...] _pits1.jpg
Mummy - aggressive, spawns at night
http://www.megomuseum.com/legend/m [...] my_art.jpg
Anubis Warrior - aggressive, spawns at night
http://www.donaldislost.com/wp-con [...] ture-1.jpg
 
Rain Forest biomes:  
Tree Frogs - passive
http://zoltantakacs.com/zt/im/scan [...] 2_v340.jpg
Lizards - passive
http://i.neoseeker.com/mgv/436139- [...] isplay.jpg  
Monkeys - passive, but they still throw ‘things’ at you, trainable
http://www.buzzle.com/img/articleI [...] 048-45.jpg
Koalas - passive, rare
http://www.stanford.edu/~jay/koalas/Koala450j.jpg
Nymph - neutral, attacks you if you attack a passive/neutral creature (so hard to find a good SFW image - they would look like sexy tree people)
http://www.josephinewall.co.uk/god [...] _nymph.jpg
Ents - neutral, drops tree things (wood, sticks, saplings), faster than cutting down trees
http://img-nex.theonering.net/imag [...] ak_big.jpg
Panther - aggressive, spawn at night
http://wwwdelivery.superstock.com/ [...] -50451.jpg
Snakes - aggressive, possibly poison, vipers
http://www.scenicreflections.com/i [...] ke_215.gif
 Apes - aggressive
http://www.awf.org/files/3972_imag [...] Watson.jpg
Constrictor snakes - aggressive, large snakes, non-poisonous
http://www.waynelynch.ca/neotropic [...] ma_101.jpg
 
Forest biomes:  
 Fruit Bats - passive
http://www.texasmonthly.com/blogs/ [...] it-bat.jpg
Possum - passive, when attacked it plays dead - hitting it while ‘dead’ will kill it, other creatures wont attack it while it is playing dead
http://blogs.995themountain.com/fi [...] possum.jpg
Beaver - passive
http://nationalzoo.si.edu/Animals/ [...] rPhoto.jpg
Red Squirrel - passive, spawns on the side of trees, can climb like spiders
http://www.internet-grocer.net/squirrel2.jpg  
Owls - passive, attacks small prey
http://www.cryptomundo.com/wp-cont [...] ed-owl.jpg
Bunnies - passive, wearable, breeds fast, make exclusive for forest biomes and plains biomes
http://svpow.files.wordpress.com/2 [...] -41946.jpg
Ducks - passive, lays eggs, make exclusive to forest biomes
http://animal.discovery.com/guides [...] d_duck.jpg
Boar - neutral, drops meat, make exclusive to forest biomes
http://www.poppyfields.net/poppy/p [...] 6_boar.jpg
Raccoons - neutral, picks up items off the ground
http://torontoist.com/attachments/ [...] accoon.jpg
 Wood Nymphs - neutral, attacks you if you attack a passive/neutral creature (so hard to find a good SFW image - they would look like sexy tree people)
http://www.josephinewall.co.uk/god [...] _nymph.jpg
Ents - neutral, drops tree things (wood, sticks, saplings), faster than cutting down trees
http://img-nex.theonering.net/imag [...] ak_big.jpg
Foxs - neutral, make them exclusive for forest biomes
http://images.nationalgeographic.c [...] 00x450.jpg
Skunks - neutral, would attack with something similar to poison, curable with water
http://animal.discovery.com/mammal [...] icture.jpg
 Moose - neutral
http://www.statesymbolsusa.org/IMAGES/Alaska/MOOSE.jpg
Bears - neutral, aggressive at night, attacks smaller creatures, make them exclusive for this biome
http://images.nationalgeographic.c [...] 00x450.jpg
Porcupines - aggressive, short agro range, easy to kill with sword, hurts to punch with bare fists
http://www.babble.com/CS/blogs/str [...] cupine.jpg
Doe/Deer - passive/aggressive respectively, usually spawn together
http://www.new-forest-national-par [...] w_deer.jpg
Wolves - aggressive, spawns at night, attacks smaller prey, make exclusive to this biome
http://www.maxwaugh.com/images/zoo02/wolf2.jpg
 
Seasonal forest biomes:  
Possum - passive, when attacked it plays dead - hitting it while ‘dead’ will kill it, other creatures wont attack it while it is playing dead
http://blogs.995themountain.com/fi [...] possum.jpg
Owls - passive, attacks small prey
http://www.cryptomundo.com/wp-cont [...] ed-owl.jpg
Bunnies - passive, wearable, breeds fast, make exclusive for forest biomes and plains biomes
http://svpow.files.wordpress.com/2 [...] -41946.jpg
Beaver - passive
http://nationalzoo.si.edu/Animals/ [...] rPhoto.jpg
Squirrel - passive, spawns on the side of trees, can climb like spiders
http://blogs.ft.com/westminster/fi [...] rel-jp.jpg
Ducks - passive, lays eggs, make exclusive to forest biomes
http://animal.discovery.com/guides [...] d_duck.jpg
Sheep - passive, sheer them for wool, comes in ranges of white-black, restrict them to only spawn in Seasonal forest and plains biomes (other functions already added with Farmcraft) (already in game)
http://www.reggie.net/photos/irela [...] ep-600.jpg
 Chickens - passive, lays eggs, restrict them to only spawn in Seasonal forest and plains biomes (other functions already added with Farmcraft) (already in game)
http://www.ifood.tv/files/images/H [...] ickens.jpg
Rooster - passive, counterpart of chickens
http://jennygardiner.net/blog/wp-c [...] ooster.jpg
Pigs - passive, restrict them to only spawn in Seasonal forest and plains biomes (other functions already added with Farmcraft) (already in game)
http://scienceblogs.com/thoughtful [...] 03/pig.jpg
Llamas - passive
http://www.ansi.okstate.edu/breeds [...] llama1.jpg
Cows - passive, milkable, restrict them to only spawn in Seasonal forest and plains biomes (other functions already added with Farmcraft) (already in game)
http://www.pollsb.com/photos/o/12838-rude_cow.jpg
Rams - neutral, counterpart to sheep
http://animalmakers.com/Catalog/images/ram_1079-01.jpg
Bulls - neutral, counterpart of cows
http://image10.webshots.com/10/0/9 [...] Vyg_ph.jpg
Boar - neutral, drops meat, make exclusive to forest biomes
http://www.poppyfields.net/poppy/p [...] 6_boar.jpg
Raccoons - neutral, picks up items off the ground
http://torontoist.com/attachments/ [...] accoon.jpg
 Skunks - neutral, would attack with something similar to poison, curable with water
http://animal.discovery.com/mammal [...] icture.jpg
Foxs - neutral, make them exclusive for forest biomes
http://images.nationalgeographic.c [...] 00x450.jpg
 Porcupines - aggressive, short agro range, easy to kill with sword, hurts to punch with bare fists
http://www.babble.com/CS/blogs/str [...] cupine.jpg
 Weasel - passive
http://thehui.files.wordpress.com/ [...] weasel.jpg
Wolves - aggressive, spawns at night, attacks smaller prey, make exclusive to this biome
http://www.maxwaugh.com/images/zoo02/wolf2.jpg
 
Savanna biomes:
Giraffes - passive
http://www.freewebs.com/nmperry06/giraffes.jpg
Zebras - passive
http://sabushveld.co.za/zebra.jpg
Buffalo - passive, spawn in packs
http://4.bp.blogspot.com/_yxE8VAdB [...] uffalo.jpg
Gazelle - passive, spawn in packs
http://static.howstuffworks.com/gi [...] -info0.gif
Flamingo - passive, spawn near water
http://www.birding.in/images/Birds [...] amingo.jpg
Hippopotamus - neutral, spawn near water, could have birds ride them
http://www.wildlife-pictures-onlin [...] o_0367.jpg
 Elephants  - neutral; possibly ridable with multiple people
 http://www.wildlife-pictures-onlin [...] 24_med.jpg
 Lions - aggressive, make them exclusive for this biome, attacks other animals
http://lionsenter.com/img/Steve-Bl [...] PH0295.jpg
Rhino - aggressive
http://www.aazkdetroit.org/images/rhino.jpg
Leopards - aggressive
http://images.nationalgeographic.c [...] 00x450.jpg
Hyena - aggressive, spawn in packs, fights lions
http://scienceblogs.com/neurotopia/hyena1.jpg
 Caiman - aggressive, spawn near water
http://nas.er.usgs.gov/XIMAGESERVE [...] 191758.gif  
 
Plains biomes:  
 Owls - passive, attacks small prey
http://www.cryptomundo.com/wp-cont [...] ed-owl.jpg
Bunnies - passive, wearable, breeds fast, make exclusive for forest biomes and plains biomes
http://svpow.files.wordpress.com/2 [...] -41946.jpg
Possum - passive, when attacked it plays dead - hitting it while ‘dead’ will kill it, other creatures wont attack it while it is playing dead
http://blogs.995themountain.com/fi [...] possum.jpg
Sheep - passive, sheer them for wool, comes in ranges of white-black, restrict them to only spawn in Seasonal forest and plains biomes (other functions already added with Farmcraft) (already in game)
http://www.reggie.net/photos/irela [...] ep-600.jpg
Chickens - passive, lays eggs, restrict them to only spawn in Seasonal forest and plains biomes (other functions already added with Farmcraft) (already in game)
http://www.ifood.tv/files/images/H [...] ickens.jpg
Rooster - passive, counterpart of chickens
http://jennygardiner.net/blog/wp-c [...] ooster.jpg
Pigs - passive, restrict them to only spawn in Seasonal forest and plains biomes (other functions already added with Farmcraft) (already in game)
http://scienceblogs.com/thoughtful [...] 03/pig.jpg
Llamas - passive
http://www.ansi.okstate.edu/breeds [...] llama1.jpg
Cows - passive, milkable, restrict them to only spawn in Seasonal forest and plains biomes (other functions already added with Farmcraft) (already in game)
http://www.pollsb.com/photos/o/12838-rude_cow.jpg
Rams - neutral, counterpart to sheep
http://animalmakers.com/Catalog/images/ram_1079-01.jpg
Bulls - neutral, counterpart of cows
http://image10.webshots.com/10/0/9 [...] Vyg_ph.jpg
Platypus - neutral, possible poison (yes they are poisonous, go look it up)
http://dcn.davis.ca.us/~platypus/images/platygeo.jpg
 Kangaroo - neutral
http://blog.bioethics.net/541447~A [...] osters.jpg
 Frilled Lizards - neutral (they flash their neck gill things at aggressors)
http://herpindiego.com/images/Aust [...] Lizard.jpg
http://www.brucemeans.com/around_w [...] ard500.jpg  
Ostrich - aggressive (they aren’t nice!)
http://images.nationalgeographic.c [...] 00x450.jpg
 
Swamp biomes:  
Frogs - passive
http://static.howstuffworks.com/gif/frog-1.jpg
Flamingo - passive, spawn near water
http://www.birding.in/images/Birds [...] amingo.jpg  
Poison dart frogs - neutral (when you attack one they try to touch you like small slimes), neon colored, possible poisonous to touch
http://farm1.static.flickr.com/193 [...] 87110a.jpg
 Alligator- aggressive, spawn in water near grass
http://animals.timduru.org/dirlist [...] igator.jpg
Snakes - aggressive, possibly poison
http://www.nearctica.com/herps/sna [...] bacur1.jpg
 
Under water:  
Regular ol’ fish - passive, they drop fresh fish
http://classes.dma.ucla.edu/Winter [...] school.jpg
Rainbow fish - passive, same as regular fish, except vibrantly colored (many varieties)
http://www.makeupgeek.com/wp-conte [...] ish-02.jpg
Jellyfish - passive, hurts to touch, hurts to punch with bare hands
http://earthguide.ucsd.edu/hughes2 [...] lyfish.gif
Bottlenose Dolphins - passive, ward off sharks, mountable?
http://www.theoriginaldolphinwatch [...] phin1b.jpg
 Whale - passive, spawn in gigantic bodies of water
http://fascinatingly.com/home/imag [...] _whale.jpg
Seahorse - passive
http://divegallery.com/seahorse_1.jpg
Giant Grouper - passive
http://www.oceanwideimages.com/ima [...] rouper.jpg
Manatee - passive (milkable sea cow? Lol)
http://www.manateetoursusa.com/ima [...] noman6.jpg
Otter - passive
http://www.visitsacandaga.com/nature/river_otter.jpg  
Plesiosaurs - passive, rare
http://www.zmescience.com/wp-conte [...] osaur1.jpg
Squid - passive (already in game)
http://files.octavarius.com/wp-con [...] -squid.jpg
Sea Turtle - passive
http://www.projects-abroad.org/_ph [...] turtle.jpg
Narwhal - neutral
http://www.cms.int/reports/small_c [...] rwhal2.jpg
Sailfish - neutral
http://myanimalblog.files.wordpres [...] .jpg?w=470
Swordfish - neutral
http://www.theultimatefishingshow. [...] marlin.jpg
Crabs - neutral, spawns on sand near water
http://www.reef.crc.org.au/researc [...] errata.jpg
 Puffer fish - neutral, possibly poisonous
http://www.manateetoursusa.com/ima [...] noman6.jpg
http://images.nationalgeographic.c [...] 00x450.jpg
Giant Squid - neutral, aggressive to whales, spawns in gigantic bodies of water
http://blog.hmns.org/wp-content/up [...] -squid.jpg
 Sharks - aggressive, attacks fish of similar or smaller size
http://sharkattackphotos.com/Bull-shark.jpg
Anglerfish - aggressive, produce light
http://www.bbc.co.uk/nature/bluepl [...] ontage.jpg
 Orcas - aggressive, spawn in gigantic bodies of water
http://www.whale-images.com/images [...] breach.jpg
 Sea snakes - aggressive, possibly poisonous
http://www.starfish.ch/photos/fish [...] rinus2.jpg
Piranhas - aggressive, spawn in group (or more like a cloud)
http://thenewsoftoday.com/wp-conte [...] havasu.jpg
Cthulhu/Kraken - aggressive, spawns in gigantic bodies of water, rare (a mix of cthulhu and kracken - would look similar to cthulhu, but act more like the kraken)
http://roberthood.net/blog/wp-cont [...] thulhu.jpg
 
Nether:  
Imps - passive, weak
http://images2.wikia.nocookie.net/ [...] _demon.jpg
Zombie Pigmen - neutral (already in game)
http://fc09.deviantart.net/fs70/f/ [...] 3374qw.jpg
Ghast - aggressive (already in game)
http://fc05.deviantart.net/fs71/i/ [...] 31bc8o.jpg
Banshee - aggressive, similar to wraiths, but able to stun players not wearing helmets for a short time (banshee screech)
http://images.elfwood.com/art/p/e/peters/banshee2.jpg
Hell hounds - aggressive, fast, produces light (one or three heads)
http://fc03.deviantart.net/fs70/f/ [...] ll1230.png
Undead Dragons - aggressive, strong and large, unable to fly or shoot fireballs (that’s the ghast’s job in the nether)
http://img227.imageshack.us/img227 [...] ich134.jpg
Various Demons - aggressive, have lots of kinds, be creative
http://carlosdev.files.wordpress.c [...] demons.jpg  
Nether Hydra - aggressive, swims in lava, two or three heads
http://www.coolstuffinc.com/images [...] 0Hydra.jpg
Charbone Archer - aggressive, tough to kill skeleton archers, possible poison
http://dungenie.com/forums/uploads [...] _max-b.png
 
All biomes Non-aggressive:  
Dogs - passive, many breeds/uses, goes along well with horses (tons of suggestions for this, I’ll keep this one short)
http://visual.merriam-webster.com/ [...] eeds_1.jpg
http://visual.merriam-webster.com/ [...] eeds_2.jpg
Horses -  passive, many breeds, tamable, ridable (already in Mo’ Creatures)
http://1.bp.blogspot.com/_k3tDzhI3 [...] 0Small.jpg
Regular ol’ Birds - passive, tamable (cageable), many types (already in Mo’ Creatures)
http://www.tpwd.state.tx.us/kids/f [...] 1birds.gif
Seagulls - passive, spawn on sandy areas near water
http://www.mccullagh.org/db9/10d-1 [...] eagull.jpg
Mice/Rats - passive, would do something bad
http://get-rid-of-pests.com/wp-con [...] s/rat1.jpg
Panda - passive, spawn near sugar cane/reeds only (can’t learn kung fu)
http://motherjones.com/files/legac [...] anda01.jpg
Jackalope - passive, extremely rare and fast, runs from player, drops diamonds
http://www.chuckstoyland.com/potpo [...] PC%201.jpg
Goats - passive, spawn at radically changing elevations on grass (mountain sides), not far from cloud line, milkable
http://www.ansi.okstate.edu/breeds [...] /BOER5.jpg  
Butterflies - passive
http://image.shutterstock.com/disp [...] 015282.jpg
Tortoise - passive (already made in Quintessential Creatures)
http://www.topnews.in/files/tortoise.jpg
Large eagles - neutral, attacks small prey, drops fish
http://thepoetsgarret.com/2009Uploads/eagle.jpg
 Gryphon - neutral, mountable, rare as a Pegasus, found on high elevations with grass
http://incendiarygames.com/wp-cont [...] ryphon.jpg
 Cats - neutral, kills mice and other small prey, tamable to follow, but not to carry anything
http://www.clowder.net/cats/SixCats.jpg
Phoenix - neutral, baby phoenixes are born from killing an adult, considered ‘dead’ when you kill a baby, babies perhaps tamable, produce light
http://www.santharia.com/pictures/ [...] hoenix.jpg
Bumble Bees - neutral, a cloud-like mob, spawns near trees
http://upload.wikimedia.org/wikipe [...] vement.jpg
 
All biomes Aggressive:  
Skeletons - aggressive, shoots arrows (already in game)
http://www.clipartpal.com/_thumbs/ [...] oodbye.png
Zombies - aggressive (already in game)
http://www.minordetailpodcast.com/images/zombies.jpg
Colored Creepers - aggressive, suicide-bomb players, colorize them to camouflage themselves to the biome they spawn in (white spawned in snow, brown spawned in desert)
http://26.media.tumblr.com/tumblr_ [...] o1_400.jpg  
Slimes - aggressive, small ones can only push you (already in game)
http://fc04.deviantart.net/fs20/f/ [...] ckbolt.jpg
Spider Jockey - Skeletons riding spiders! Need more mob combos (surprised I couldn’t find any good art of this) (already in game)
http://s3.amazonaws.com/readers/20 [...] ider_1.png
Bobcat - aggressive, spawn at radically changing elevations on grass (mountain sides), not far from cloud line
http://blog.ctnews.com/connecticut [...] cat_04.jpg
Vampires - aggressive, Killed with wooden or golden swords (wooden because of relation to stake, golden for convenience as it can also kill werewolf), aggressive to werewolves, should have long dark capes to make them identifiable, and ‘glide’ instead of walking, spawn at night
http://boozers.fortunecity.com/cam [...] ampire.jpg
Werewolves - aggressive, killed with iron or golden swords (iron, because it looks closer to silver than gold, and gold, because that’s what DrZhark chose), aggressive to vampires, (already in Mo’ Creatures)
http://www.werewolves.com/wordpres [...] /spell.jpg
Dragons - aggressive, extremely rare, spawns at or above cloud level (the fancy details cloud level), requirement of spawning on grass, flies, and shoots fireballs like ghasts, possible different types, NOT tamable (they should be huge)
http://www.jewcy.com/wp-content/up [...] dragon.jpg
Gargoyles - aggressive, they would stay still like stone until you approach them, spawn at night
http://images.epilogue.net/users/abi/the-Gargoyle.jpg
Golems - aggressive, possible to change them to passive, or make them attack other creatures
http://www.itusozluk.com/img.php/d [...] 7108/golem
Minotaurs - aggressive, spawn at night (spawn in your maze)
http://fc04.deviantart.net/fs17/f/ [...] y_orgo.jpg
Troll - aggressive, thinner than ogres, throw rocks, spawn at night (or maybe hide under your bridge - don’t feed them) (already made in Quintessential Creatures)
http://www.rpgminiatures.com/acata [...] nTroll.jpg
Wraiths - aggressive, flies (already in Mo’ Creatures)
http://images.wikia.com/forgottenr [...] wraith.jpg
Ogres - aggressive, destroyed blocks, tough mob (already in Mo’ Creatures)
http://fc04.deviantart.net/fs40/f/ [...] abrush.jpg  
Wasps - aggressive, cloud-like mob, spawns near trees, careful not to mistake them with bumble bees (could also be called killer bees)
http://www.naturephoto-cz.com/phot [...] p-0071.jpg
Mimic Chest - aggressive, trick chests found in dungeons, right clicking them wakes them
http://image.mabinogi.com/_imgs/85 [...] ic_001.gif
 
 Mobs not to apply:  
Goblins - would be better saved for mob villages
Pigmen - would be better saved for mob villages (model already in game)
 
For anything that says ‘possible poison’ or something of that nature, I would suggest poison be like a short term fire effect (screen would go a hue of green, and/or your character look a hue of green).  Anything that can poison would drop poison sacks, which in turn could be used to make antidotes or to poison your weapons.
 
PS.  Don’t quote this entire monster of a post.  Cut the list out or use this link to the post instead!
http://www.minecraftforum.net/view [...] 4#p2364314