The Collaborative International Dictionary of English v.0.48:

Sea elephant \Sea" el"e*phant\ (s[=e]" [=e]"[-e]*fant). (Zool.)
   A very large seal (Macrorhinus proboscideus) of the
   Antarctic seas, much hunted for its oil. It sometimes attains
   a length of thirty feet, and is remarkable for the
   prolongation of the nose of the adult male into an erectile
   elastic proboscis, about a foot in length. Another species of
   smaller size (Macrorhinus angustirostris) occurs on the
   coast of Lower California, but is now nearly extinct.
   [1913 Webster]