Source : Webster's Revised Unabridged Dictionary (1913)

Sea horse \Sea" horse`\
   1. A fabulous creature, half horse and half fish, represented
      in classic mythology as driven by sea dogs or ridden by
      the Nereids. It is also depicted in heraldry. See
      {Hippocampus}.

   2. (Zo["o]l.)
      (a) The walrus.
      (b) Any fish of the genus Hippocampus.

   Note: In a passage of Dryden's, the word is supposed to refer
         to the hippopotamus.

Source : WordNet®

sea horse
     n 1: either of two large northern marine mammals having ivory
          tusks and tough hide over thick blubber [syn: {walrus},
          {seahorse}]
     2: small fish with horselike heads bent sharply downward and
        curled tails; swim in upright position [syn: {seahorse}]