The Collaborative International Dictionary of English v.0.48:

Longhorn \Long"horn`\, n.
   1. (Zool.) A long-horned animal, as a cow, goat, or beetle.
      See Long-horned.
      [1913 Webster]

   2. The Texas longhorn.
      [PJC]

WordNet (r) 3.0 (2006):

longhorn
    n 1: long-horned beef cattle formerly common in southwestern
         United States [syn: longhorn, Texas longhorn]