Ascaphidae (Tailed frogs)

The tailed frogs are two species of frogs in the genus Ascaphus, the only taxon in the family Ascaphidae /æˈskæfɪdiː/. The “tail” in the name is actually an extension of the male cloaca. The tail is one of two distinctive anatomical features adapting the species to life in fast-flowing streams. These are the only North American frog species that reproduce by internal fertilization.

— Wikipedia