So he's not from the Sacred Grounds, and there are stronger bats... Hmm. Probably not from Grasstown, and I don't remember there being any bats in the waterway or the labyrinth. So either he's from Grasstown, or he isn't from anyplace that we know of.
I don't know if I've mentioned this theory yet, but he could be a human or mimiga turned into a bat by the doctor as a practical joke or something. It's possible.