A Mustang is a free-roaming feral horse of the North American west. It first descended from horses brought to the Americas by the Spanish.The name "Mustang" is also popular for high-performance products and sports mascots.

In 1971, the United States Congress recognized Mustangs as "living symbols of the historic and pioneer spirit of the West, which continue to contribute to the diversity of life forms within the Nation and enrich the lives of the American people." Today, Mustang herds vary in the degree to which they can be traced to original Iberian horses. Some contain a greater genetic mixture of ranch stock and more recent breed releases, others are relatively unchanged from the original Iberian stock, most strongly represented in the most isolated populations.

The first Mustangs descended from Iberian horses brought to Mexico and Florida. Most of these horses were of Andalusian, Arabian and Barb ancestry. Some of these horses escaped or were stolen by Native Americans, and rapidly spread throughout western North America.

Native Americans quickly adopted the horse as a primary means of transportation. Interestingly, in light of the horse's prehistoric existence in the Americas, many Indian myths and stories about the arrival of horses claimed that "the grass remembered" them. Horses replaced the dog as a travois puller and greatly improved success in battles, trade, and hunts, particularly buffalo hunts. Many tribes bred their horses carefully to improve them for their purposes. Among the most capable horse-breeding people of North America were the Comanche, the Shoshoni, and the Nez Perce. The latter in particular became master horse breeders, and developed one of the first truly American breeds: the Appaloosa. Most other tribes did not practice extensive amounts of selective breeding, though they sought out desirable horses through capture, trade and theft; plus quickly traded away or otherwise eliminated those with undesirable traits.



Starting in the colonial era and continuing with the westward expansion of the 1800s, horses belonging to explorers, traders and settlers that escaped or were purposely released joined the gene pool of Spanish-descended herds. It was also common practice for western ranchers to release their horses to locate forage for themselves in the winter and then recapture them, as well as any additional mustangs, in the spring. Some ranchers also attempted to "improve" wild herds by shooting the dominant stallions and replacing them with pedigreed animals.

In some modern mustang herds there is still clear influence of other domesticated horses being added to feral herds. Some herds show clear influence of Thoroughbred or other light racehorse-type stallions being turned into the wild herds, a process that also led in part to the creation of the American Quarter Horse. Others show the addition of heavy draft horse breeding, where farm horses were turned into wild herds in the wake of the Dawes Act, in a misguided attempt to create workhorses and force Indian people to become farmers. Other, more isolated herds, retain a strong influence of original Spanish stock.