The Collaborative International Dictionary of English v.0.48:

Sea unicorn \Sea" u"ni*corn\ (Zool.)
   The narwhal.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.] (Zool.)
   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.
   [1913 Webster]