The Collaborative International Dictionary of English v.0.48:

Sheepshead \Sheeps"head`\, n. [So called because of the fancied
   resemblance of its head and front teeth to those of a sheep.]
   (Zool.)
   A large and valuable sparoid food fish (Archosargus
   probatocephalus syn. Diplodus probatocephalus) found on
   the Atlantic coast of the United States. It often weighs from
   ten to twelve pounds.
   [1913 Webster]

   Note: The name is also locally, in a loose way, applied to
         various other fishes, as the butterfish, the
         fresh-water drumfish, the parrot fish, the porgy, and
         the moonfish.
         [1913 Webster]

WordNet (r) 3.0 (2006):

sheepshead
    n 1: large (up to 20 lbs) food fish of the eastern coast of the
         United States and Mexico [syn: sheepshead, Archosargus
         probatocephalus]