mustang [Sp. mesteño = a stray], small feral horse of the W United States. Mustangs are descended from escaped Native American horses, which in turn were descended from horses of North African blood, brought to the New World by the Spanish c.1500. Mustangs have evolved their own distinguishing traits: they are small, swift, hardy, and intelligent—well suited to plains conditions. As ranching expanded in North America, cowboys began rounding up mustangs for use as cow ponies. Hence, in the terminology of ranchers, mustang often refers to a cow pony of feral stock, and the term bronco is used for an untamed mustang. A cayuse (after the Cayuse of the NE United States) is a domestic Native American horse. Although the mustang, which has spent many generations in the wild, is somewhat different from the cayuse, the terms are sometimes used interchangeably. Cow ponies of mustang descent have been crossed with other breeds of horse, so that all horses of the W United States probably have mustang blood. The mustang, a variety of Equus caballus, is classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Perissodactyla, family Equidae.