[Rolando]

it may be mostly cosmetic, so unnatural features is all you need, if it is unnatural at all. In some campaign it may be all natural to have a fox head and a human body.

If you want something like the old teenage mutant ninja turtles RPG, you just decide what special characteristics your character have, some may have the fur perk, other armored scales (DR), night vision, claws, etc.

you imitate the animal characteristics you want your character to have as advantages and disadvantages.