What is "sea unicorn"

Narwhal \Nar"whal\ (n[aum]r"hw[.a]l), n. [Sw. or Dan. narvhal; akin to Icel. n[=a]hvalr, and E. whale. the first syllable is perh. from Icel. n[=a]r corpse, dead body, in allusion to the whitish color its skin. See Whale.] [Written also narwhale and narwal.] (Zo["o]l.) 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.

n. The narwhal.