To be completely honest I agree that SW and MO creatures weren't as consistent as the earlier ones. Some were fantastic whereas others in my opinion just didn't fit in.
I don't think it's so much about Oddworld not having variety, more about the atmosphere of the game. AO and (to a lesser extent AE) were comparatively darker than the newer games, and you expect the animals etc. to be more monstrous as a result.
I really loved the foreboding tension of the earlier games, and would personally prefer the creatures to be creepy as hell! Oddworld was described back then as a 'world where everything wants to eat you'. This in mind, I don't think chippunks or meeps really fit the bill

Sure, there would be creatures like this on a varied planet, but they really don't add anything to the mood of the game. If anything, they detract from it.
I suppose it depends how they are handled. I always wished in SW that the dangerous live ammo creatures (especially fuzzles) would attack you in packs if you got too close, before zapping them. We're supposed to believe they are deadly, but they just hop around without any reaction!