The Collaborative International Dictionary of English v.0.48:

Broncho \Bron"cho\, n. [Sp. bronco rough, wild.]
   A native or a Mexican horse of small size. [Western U.S.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

broncho
    n 1: an unbroken or imperfectly broken mustang [syn: bronco,
         bronc, broncho]