Which whale is the only living species in the genus Monodon?

The whale that is the only living species in the genus Monodon is the narwhal.

The narwhal's scientific name is Monodon monoceros, and it is the sole living member of the genus Monodon. It is an Arctic toothed whale, not a true dolphin or a baleen whale. Its closest living relative is the beluga; together, narwhals and belugas make up the two living species in the family Monodontidae.

The famous tusk is usually found in males and is actually an elongated left canine tooth. It can grow several metres long and has a spiral pattern. Females may also develop a tusk, but this is much less common. Narwhals lack a conventional dorsal fin, instead having a low ridge that helps them move beneath sea ice.

The genus name is easy to confuse with the species name: Monodon is the genus, while monoceros identifies the species. “Beluga” is not correct here because the beluga belongs to a different genus, Delphinapterus. The narwhal's name ultimately comes from Old Norse and is linked to its mottled, corpse-like coloration.