National Geographic has an article about a "hotbed of deception" in the natural world. It involves the genitals of a small, reddish-brown parasite called the bat bug. In order to protect themselves from the unwanted advances of male members of the species, female bat bugs have evolved a region on their body similar to a fake genital:
Researchers have long known that male bat bugs ignore females' conventional parts and instead use their sharp penises to stab the females' abdomens, injecting sperm directly into the bloodstream. So the females evolved a defense: structures called paragenitals that guide a male's needle-like member into a spongy reservoir of immune cells.
It turns out that male bat bugs sport a similar fake genital area, because males often perform "the same injurious sexual acts on other males." All in all, the sex life of the bat bug sounds like a rather unpleasant affair.