Narwhal n. : An arctic cetacean (Monodon monocerous), about twenty feet long. The male usually has one long, twisted, pointed canine tooth, or tusk projecting forward from the upper jaw like a horn, whence it is called also sea unicorn, unicorn fish, and unicorn whale. Sometimes two horns are developed, side by side.

Example:
When did the Narwhal first exist on earth?

Taxonomy
narwhal, tusk, narwhal tusk, Monodon monoceros, whales, Jørgensen, species, Heide, Narwhal class submarine, dives, Greenland, Narwhal Whales, horn, free encyclopedia, populations, diet, depths, weigh.

Defination from the web:
... Found primarily in Canadian Arctic and Greenlandic waters rarely south of 65°N latitude, the narwhal is a uniquely specialized Arctic predator. ...