They should be able to take the form of any small or medium sized animal, which includes the following in NWN (leaving off Dire variants):
- Black bear M
- Boar M
- Black/gold/hybrid Cormyrian cobra / Great cobra / Spitting cobra (just "snake"?) M
- Cougar / Crag cat M
- Cow* M
- Deer M
- Dog* M
- Falcon S
- Horse* M
- Jaguar M
- Leopard M
- Lion M
- Ox* M
- Panther M
- White stag M
- Wolf M
Not all of the creatures on the list would work well with the theme in my opinion, but it would be great to be able to turn into at least a couple of basic types. As an imp character, personally, fey seems to be the only shape worth even trying to disguise as, but it suffers from the OOC declaration of being polymorphed on every transition.
In the PnP rules, imps and druids (at least at low level) can't turn into tiny animals, but it seems to me like there are some interesting ones on the list there too:
- Badger T
- Bat T
- Chicken* T
- Parrot T
- Penguin T
- Rat T
- Raven T
- Seagull T