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]

WordNet (r) 3.0 (2006):

unicorn
    n 1: an imaginary creature represented as a white horse with a
         long horn growing from its forehead

Moby Thesaurus II by Grady Ward, 1.0:

154 Moby Thesaurus words for "unicorn":
   Argus, Briareus, Cerberus, Charybdis, Cyclops, Echidna, Gorgon,
   Harpy, Hydra, Loch Ness monster, Medusa, Minotaur, Pegasus, Python,
   Scylla, Sphinx, Talos, Typhon, achievement, alerion, animal charge,
   annulet, argent, armorial bearings, armory, arms, azure, bandeau,
   bar, bar sinister, baton, bearings, bend, bend sinister, billet,
   blazon, blazonry, bordure, broad arrow, cadency mark, canton,
   centaur, chaplet, charge, chevron, chief, chimera, coat of arms,
   cockatrice, coronet, crescent, crest, cross, cross moline, crown,
   device, difference, differencing, dragon, drake, eagle, equipage,
   ermine, ermines, erminites, erminois, escutcheon, falcon, fess,
   fess point, field, file, flanch, fleur-de-lis, four-in-hand, fret,
   fur, fusil, garland, griffin, gules, gyron, hatchment, helmet,
   heraldic device, hippocampus, honor point, impalement, impaling,
   inescutcheon, label, lion, lozenge, mantling, marshaling, martlet,
   mascle, mermaid, merman, metal, motto, mullet, nixie,
   nombril point, octofoil, ogre, ogress, or, ordinary, orle, pair,
   pale, paly, pean, pheon, purpure, quarter, quartering, randem, rig,
   roc, rose, sable, salamander, saltire, satyr, scutcheon, sea horse,
   sea serpent, shield, siren, span, spike, spike team, spread eagle,
   subordinary, tandem, team, tenne, three-up, tincture, torse,
   tressure, troll, turnout, vair, vampire, vert, werewolf, windigo,
   wreath, xiphopagus, yale, zombie



Easton's 1897 Bible Dictionary:

Unicorn
   described as an animal of great ferocity and strength (Num.
   23:22, R.V., "wild ox," marg., "ox-antelope;" 24:8; Isa. 34:7,
   R.V., "wild oxen"), and untamable (Job 39:9). It was in reality
   a two-horned animal; but the exact reference of the word so
   rendered (reem) is doubtful. Some have supposed it to be the
   buffalo; others, the white antelope, called by the Arabs rim.
   Most probably, however, the word denotes the Bos primigenius
   ("primitive ox"), which is now extinct all over the world. This
   was the auerochs of the Germans, and the urus described by
   Caesar (Gal. Bel., vi.28) as inhabiting the Hercynian forest.
   The word thus rendered has been found in an Assyrian inscription
   written over the wild ox or bison, which some also suppose to be
   the animal intended (comp. Deut. 33:17; Ps. 22:21; 29:6; 92:10).