It's possible, ShadowKittie is right, I've tried a texturepack with mobs with multiple kinds of textures, and when I looked inside of it, it had a folder named "mcpatcher", textures like blocks can also have random textures, try looking up the texture pack "ChromaHills".

To clarify, your filepath for the randommobs textures should look something like this: \assets\minecraft\mcpatcher\mob
The contents of the "mob" folder should be same as for the \assets\minecraft\textures\entity folder, except the mob textures should be labelled "creeper2.png," "chicken3.png," etc. What I mean by that is that mobs such as cows and endermen that have their own folders within \textures\entity should have a matching folder within \mcpatcher\mob.
This confused me a bit at first until I downloaded a resource pack that supports random mobs and had a look for myself. Hope this helps.

I don't really see it changing anything about the games except the colors of things...

Regardless,

Sorry that my help has mixed reviews, but I do hope it helps those who wanted the answers as much as I did. x3

You can, with the steps I stated; you'll just need either optifine OR mcpatcher.

I don't think that you can do it in vanilla

Found it!...I think.

The texture pack Craftee pack advertized random mobs, and they worked for me. I opened up the texture pack, and they have placed a folder in there titled 'mcpatcher', with everything you'll need as a template. The mob number system has not changed.

I hope this helps.

I am searching as well, with little luck. If I find an answer, I will most certainly post it here.

So I found out that optifine will allow for random mod support does anyone know where to put the files?... like is it the chicken2.png like in the earlier versions or shall the destination be somewhere else. Thanks in advanced

Can't do it in Vanilla.

You Can't just add things like that...
Quote from Wiki:
Resource packs eventually allow players to customize music, sounds,language files, end credits, and fonts without any code modification