The Collaborative International Dictionary of English v.0.48:

Mustang \Mus"tang\, n. [Sp. muste[~n]o belonging to the
   graziers, strayed, wild.] (Zool.)
   The half-wild horse of the plains in Mexico, California, etc.
   It is small, hardy, and easily sustained.
   [1913 Webster]

   Mustard grape (Bot.), a species of grape (Vitis
      candicans), native in Arkansas and Texas. The berries are
      small, light-colored, with an acid skin and a sweet pulp.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mustang
    n 1: small hardy range horse of the western plains descended
         from horses brought by the Spanish

The Devil's Dictionary (1881-1906):

MUSTANG, n.  An indocile horse of the western plains.  In English
society, the American wife of an English nobleman.