Which Arctic whale shares the family Monodontidae with the narwhal?

The Arctic whale that shares the family Monodontidae with the narwhal is the beluga whale.

Monodontidae contains two living species: the beluga (Delphinapterus leucas) and the narwhal (Monodon monoceros). Both are toothed whales adapted to Arctic and sub-Arctic waters, where they navigate among sea ice and coastal channels.

Belugas are recognizable for their white adult coloration, rounded forehead, and lack of a true dorsal fin. Narwhals are famous for the long tusk that usually develops from an upper tooth, especially in males. Despite their very different appearances, the two animals share important traits, including a flexible neck and a prominent melon used in sound production and echolocation.

A common mix-up is treating the beluga as a dolphin or porpoise because of its rounded body and small size. It is a whale, as is the narwhal. Sei, gray, and minke whales belong to different families and are not monodontids.