The Collaborative International Dictionary of English v.0.48:

Pronghorn \Prong"horn`\, n. (Zool.)
   An American antelope (Antilocapra Americana), native of the
   plain near the Rocky Mountains. The upper parts are mostly
   yellowish brown; the under parts, the sides of the head and
   throat, and the buttocks, are white. The horny sheath of the
   horns is shed annually. Called also cabr['e]e, cabut,
   prongbuck, and pronghorned antelope.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pronghorn
    n 1: fleet antelope-like ruminant of western North American
         plains with small branched horns [syn: pronghorn,
         prongbuck, pronghorn antelope, American antelope,
         Antilocapra americana]