I've been exploring similar ideas for my homebrewed system for Mutant Town. For what it's worth, here's how I handled it:
Players can choose which kinds of animals to be (or pick one entirely at random; it's up to the player). But I still want to keep the system simple, so rather than go through the trouble of assigning stats to every possible kind of animal, instead I go by a few Size Classes. They're size categories for characters to assume, and they come with their own bonuses and penalties:
Size Classes [available to PCs]: - Small (+2 Speed, +1 Vitality, -1 Defense) 1' - 3' tall.
- Medium (Regular human size; no bonuses) 3'-7' tall.
- Large (+2 Offense, +1 Defense, -1 Speed) '7-9' tall.
Size Classes [unavailable to PCs]: - Tiny (+3 Speed, +2 Vitality, -3 Defense) Insect-sized.
- Gigantic (+3 Offense, +2 Defense, -3 Speed) Monster-sized.
The reason I decided to do it this way -- besides reducing the potential workload -- was because I noticed that TMNT characters seem to come in size categories that didn't necessarily reflect their human or animal forms. In Mutant Town, for instance, we see platypus, porcupine, hawk, shark, hippo, and even a goose mutant all in what would be the Large Size Class. There was an ant mutant and a stick insect mutant among the many students at the dojo, and they were Medium size. Then there were various characters (usually children) in the Small category.
I thought that was fascinating, because there didn't seem to be any rhyme or reason to whether an animal/human hybrid would be tiny or a bruiser. They seemed as random as the Bio-E system in
Other Strangeness. So I just figured the size class would be the important part in terms of stats, rather than the specific animal chosen.
Other stat bonuses come from Group Affiliation and Personality Traits. Weapons act as bonuses to damage, and they take the form of Weapon Groups (Blunt, Edged, Ninja, Ranged groups so far). This way I don't have to stat out and differentiate all the many types of weapons.
Just thought I'd share some design ideas that I've been working on.