The Collaborative International Dictionary of English v.0.48:

Hippocampus \Hip`po*cam"pus\, n. [L., the sea horse, Gr. ? a
   hippocampus (in senses 1 and 2); "i`ppos horse + ? to bend.]
   1. (Class. Myth.) A fabulous monster, with the head and fore
      quarters of a horse joined to the tail of a dolphin or
      other fish (Hippocampus brevirostris), -- seen in
      Pompeian paintings, attached to the chariot of Neptune.
      --Fairholt.
      [1913 Webster]

   2. (Zool.) A genus of lophobranch fishes of several species
      in which the head and neck have some resemblance to those
      of a horse; -- called also sea horse.
      [1913 Webster]

   Note: They swim slowly, in an erect position, and often cling
         to seaweeds by means of the incurved prehensile tail.
         The male has a ventral pouch, in which it carries the
         eggs till hatched.
         [1913 Webster]

   3. (Zool.) A name applied to either of two ridges of white
      matter in each lateral ventricle of the brain. The larger
      is called hippocampus major or simply hippocampus. The
      smaller, hippocampus minor, is called also ergot and
      calcar.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Zool.)
      (a) The walrus.
      (b) Any fish of the genus Hippocampus.
          [1913 Webster]

   Note: In a passage of Dryden's, the word is supposed to refer
         to the hippopotamus.
         [1913 Webster]

WordNet (r) 3.0 (2006):

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