This is by no means a new idea, but the synthesis of several ideas that has been suggested many times over the years. Like horses and rabbits, each mob could have several appearance variants, and those variants could be based on the biome in which it generates.

Here are the variants I feel would work best (with some images from real life or existing mods.) I'm not necessarily suggesting that we need all of these, but I think many of these would be an excellent addition to 1.10:

Hostile mobs:

Zombie:

• Mummy – Desert
• Swamp monster – Swamp (basically just a grosser version of a zombie)
• Frozen Zombie – Ice Plains

Spider:

• Scorpion – Desert
• Tarantula – Jungle
• Black Widow – Dark Forest
• Spider Crab – Beach (maybe…)

Creeper:

• Add variants to match the prevalent color/texture of certain biomes, like sand in deserts or stone in extreme hills (e.g. http://ift.tt/24OOADc)

Passive mobs:

Sheep:

• Bighorn Sheep – Extreme Hills

Cow:

• Cape Buffalo – Savanna
• Yak – Extreme Hills, Taiga, Ice Plains

Pig:

• Boar – Forest
• Warthog – Savanna
• Peccary – Jungle

Chicken:

• Pheasant – Oak forest
• Roadrunner – Desert
• Red Junglefowl – Jungle
• Spruce Grouse – Spruce forest
• Moorhen – Swamp

Wolf:

• Hyena – Savanna
• Coyote – Desert
• Timber Wolf – Extreme Hills, Taiga
• Arctic Wolf – Ice Plains

Ocelot:

• Bobcat – Desert
• Lynx – Extreme Hills, Taiga
• Serval – Savanna

Donkey:

• Zebra – Savanna

This feature would add a lot to the game without changing any existing game mechanics. Collecting all the mob variants would be a fun incentive for exploring other biomes, and it just adds a little more "life" to the game. It could easily be backwards compatible with older resource packs (e.g. use the Donkey model as fallback for Zebra), and programmatically speaking, it's fairly trivial to tag and render an entity based on it's spawn biome. This would be a relatively low hanging fruit for 1.10, yet it would do a lot to make the game feel new and exciting.

Just like with horses and rabbits, passive mobs could be bread with any other variant of the same animal, and the offspring would be one or the other, assigned randomly. Drops, experience, AI, and other mechanics would be the same for each variant – the variant would only change the mobs' appearance.