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
    n 1: a river that flows from central Wyoming to the Yellowstone
         River in southern Montana [syn: Bighorn, Bighorn River]
    2: 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]