There is a truly dreadful, assembly-line-produced series of books about fairies that Filia was addicted to a few years ago. The fairies are all female (except for the King), and their adversaries, the goblins, are all male. The whole series is basically a thinly-disguised "girls against the boys" story, in which the boys always lose because they quarrel amongst themselves and cannot for the life of themselves cooperate.

But anyway, I came up with the theory that the fairies and goblins are the females and males of a single species.