I am sure this would be easy to impliment due to the fact that all the critter parts have to be located in the game somewhere, its just a matter of getting access to them.
The problem is that there are several critter "parts": the mesh, the skeleton, the animations, and the skins. These parts cannot generally be swapped and recombined to create new creatures.
Right now it is possible to modify the skins of animals and objects, since these are image files that are easily edited. Now, if there was a way to edit the other parts, it would be a boon for modders.