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]

The Collaborative International Dictionary of English v.0.48:

Unicorn \U"ni*corn\, n. [OE. unicorne, F. unicorne, L. unicornis
   one-horned, having a single horn; unus one + cornu a horn;
   cf. L. unicornuus a unicorn. See One, and Horn.]
   1. A fabulous animal with one horn; the monoceros; -- often
      represented in heraldry as a supporter.
      [1913 Webster]

   2. A two-horned animal of some unknown kind, so called in the
      Authorized Version of the Scriptures.
      [1913 Webster]

            Canst thou bind the unicorn with his band in the
            furrow?                               --Job xxxix.
                                                  10.
      [1913 Webster]

   Note: The unicorn mentioned in the Scripture was probably the
         urus. See the Note under Reem.
         [1913 Webster]

   3. (Zool.)
      (a) Any large beetle having a hornlike prominence on the
          head or prothorax.
      (b) The larva of a unicorn moth.
          [1913 Webster]

   4. (Zool.) The kamichi; -- called also unicorn bird.
      [1913 Webster]

   5. (Mil.) A howitzer. [Obs.]
      [1913 Webster]

   Fossil unicorn, or Fossil unicorn's horn (Med.), a
      substance formerly of great repute in medicine; -- named
      from having been supposed to be the bone or the horn of
      the unicorn.

   Unicorn fish, Unicorn whale (Zool.), the narwhal.

   Unicorn moth (Zool.), a notodontian moth (Coelodasys
      unicornis) whose caterpillar has a prominent horn on its
      back; -- called also unicorn prominent.

   Unicorn root (Bot.), a name of two North American plants,
      the yellow-flowered colicroot (Aletris farinosa) and the
      blazing star (Chamaelirium luteum). Both are used in
      medicine.

   Unicorn shell (Zool.), any one of several species of marine
      gastropods having a prominent spine on the lip of the
      shell. Most of them belong to the genera Monoceros and
      Leucozonia.
      [1913 Webster]