Hmm, are you suggesting we should add The Thing versions for wildlife? Because I love The Thing, and think Cataclysm could always use more biohorror. I like the idea of some more ambushing type monsters too.
Fortunately, monsters transforming is already supported, so no hard coding. The large and medium variants can be roughly based on the existing zombie variants, but with some much more flavorful gaping maws and tentacles, etc. For the smaller variants like squirrels and rats, I’m not quite sure how to keep them threatening, but I think a special ability rather than raw attack makes more sense. Don’t just wanna have a killer rabbit, after all.
Lurking mutant farm animals could be interesting too. One sheep standing off in a corner, all the others fleeing it, a suspicious bloodstain…