The Collaborative International Dictionary of English v.0.48:

Walrus \Wal"rus\, n. [D. walrus; of Scand. origin; cf. Dan
   valros, Sw. vallross, Norw. hvalros; literally, whale horse;
   akin to Icel. hrosshvalr, AS. horshwael. See Whale, and
   Horse.] (Zool.)
   A very large marine mammal (Trichecus rosmarus) of the Seal
   family, native of the Arctic Ocean. The male has long and
   powerful tusks descending from the upper jaw. It uses these
   in procuring food and in fighting. It is hunted for its oil,
   ivory, and skin. It feeds largely on mollusks. Called also
   morse.
   [1913 Webster]

   Note: The walrus of the North Pacific and Behring Strait
         (Trichecus obesus) is regarded by some as a distinct
         species, by others as a variety of the common walrus.
         [1913 Webster]

WordNet (r) 3.0 (2006):

walrus
    n 1: either of two large northern marine mammals having ivory
         tusks and tough hide over thick blubber [syn: walrus,
         seahorse, sea horse]