The Collaborative International Dictionary of English v.0.48:

Pygarg \Py"garg\, Pygargus \Py*gar"gus\, [L. pygargus, Gr. ?,
   literally, white rump; ? the rump + white: cf. F. pygargue.]
   1. (Zool.) A quadruped, probably the addax, an antelope
      having a white rump. --Deut. xiv. 5.
      [1913 Webster]

   2. (Zool.)
      (a) The female of the hen harrier.
      (b) The sea eagle.
          [1913 Webster]

Easton's 1897 Bible Dictionary:

Pygarg
   Heb. dishon, "springing", (Deut. 14:5), one of the animals
   permitted for food. It is supposed to be the Antelope addax. It
   is described as "a large animal, over 3 1/2 feet high at the
   shoulder, and, with its gently-twisted horns, 2 1/2 feet long.
   Its colour is pure white, with the exception of a short black
   mane, and a tinge of tawny on the shoulders and back.",
   Tristram's Natural History.