mus·tang 
(mŭstăng′)
(mŭstăng′)n.
A small, hardy wild horse of the North American plains, descended from horses brought to America by Spanish explorers and European settlers.
[American Spanish mesteño, mestengo, stray animal, from Old Spanish, from mesta, association of livestock owners, from Medieval Latin (animālia) mixta, assorted (animals), from Latin, neuter pl. past participle of miscēre, to mix; see meik- in the Appendix of Indo-European roots.]
mustang