It would depend heavily upon the animal. Sims animals aren't using anything like Daz 3D's genesis base, let alone dynamic rigging. Some animals would work, of course, but stretching a large Dog into a properly sized horse or an adult cow (as opposed to a calf), for example, is probably going to result in a lot of mesh and rigging issues. Now, if you kept it say... sheep or goat sized, then it would probably work. In the same way, you could probably use cats and small dogs for Rabbits and skunks.
But they'd almost certainly need new rigging, or at least new joint parameters, as many of these animals move differently too. And once you get to that point, the question becomes which is the more efficient route... starting fresh with a new mesh and new rigging, or altering the old to work. Depending on the project, sometimes starting fresh is actually the better route.
