The Collaborative International Dictionary of English v.0.48:

Bighorn \Big"horn`\, n. (Zool.)
   The Rocky Mountain sheep (Ovis montana or Caprovis
   montana); called also bighorn sheep.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bighorn sheep
    n 1: wild sheep of mountainous regions of western North America
         having massive curled horns [syn: bighorn, bighorn
         sheep, cimarron, Rocky Mountain bighorn, Rocky
         Mountain sheep, Ovis canadensis]