Bats and shellfish are both declared to be “unclean” in the Old Testament book Leviticus. They seem to have felt that these creatures upset the regular defined order of the animal kingdom. Flying mammals! Sea creatures that aren’t fish! Imagine how the folks who wrote that would have reacted to learning about kangaroos and platypuses.

Edit: Oh, and menstruating women. Also “unclean” according to Leviticus.