Source : Webster's Revised Unabridged Dictionary (1913)

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

   {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.

Source : WordNet®

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