Today, the only true wild horse is the Przewalski's Horse, native to Mongolia. However, the horse family Equidae and the genus Equus evolved in North America. Studies using ancient DNA as well as DNA of recent individuals shows there once were two closely related horse species in North America, the "wild horse" (Equus ferus) and the "Stilt-legged Horse;" which is taxonomically assigned to various names. Thus, primitive horses lived in North America in prehistoric times. However, the entire equus genus died out at the end of the last ice age around 10-12,000 years ago, possibly due to a changing climate or the impact of newly arrived human hunters. Thus at the beginning of the Columbian Exchange, there were no equids in the Americas at all. Horses first returned with the Conquistadors, beginning with Columbus, who imported horses from Spain to the West Indies on his second voyage in 1493. Domesticated horses came to the mainland with the arrival of Cortés in 1519.
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. Horses replaced the dog as a travois puller and greatly improved success in battles, trade, and hunts, particularly bison hunts.
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.
By 1900 North America had an estimated two million free-roaming horses. Since 1900, the Mustang population has been reduced drastically. Mustangs were viewed as a resource that could be captured and used or sold (especially for military use) or slaughtered for food, especially pet food. The controversial practice of mustanging was dramatized in the John Huston film The Misfits, and the abuses linked to certain capture methods, including hunting from airplanes and poisoning, led to the first federal wild free-roaming horse protection law in 1959. Protection was increased further by the Wild Free-Roaming Horse and Burro Act of 1971.
The 1971 Act provided for protection of certain previously established herds of horses and burros. Today, the United States Forest Service administers 37 wild horse or burro territories in several western states.
Historically, many of the Native American 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 last 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, and quickly traded away or otherwise eliminated those with undesirable traits.
In some modern mustang herds there is clear evidence of other domesticated horse breeds having become intermixed with feral herds. Some herds show the signs of the introduction of Thoroughbred or other light racehorse-types into herds, a process that also led in part to the creation of the American Quarter Horse. Other herds show signs of the intermixing of heavy draft horse breeds turned loose in an attempt to create work horses. Other, more isolated herds, retain a strong influence of original Spanish stock.
Some breeders of domestic horses consider the Mustang herds of the west to be inbred and of inferior quality. However, supporters of the Mustang argue that the animals are merely small due to their harsh living conditions and that natural selection has eliminated many traits that lead to weakness or inferiority. Some mustang supporters also maintain that some "inbreeding" actually concentrates the traits of hardiness and durability, making the mustang a valuable genetic resource. Regardless of these debates, the Mustang of the modern west has several different breeding populations today which are genetically isolated from one another and thus have distinct traits traceable to particular herds. These 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.
Two researchers have advanced an argument that Mustangs should be legally classified as "wild" rather than "feral." They argue that, due to the presence of Equus ferus ferus on the North American continent till the end of the Pleistocene era, horses were once native animals and should still be considered as native animals, and therefore defined as "wild," and not viewed as an exotic species that draws resources and attention away from true native species.
Mustang in NW Nevada mountains
Today, free-roaming horses are protected under United States law, but have disappeared from several states where there were once established populations. A few hundred free-roaming horses survive in Alberta and British Columbia. The BLM considers 27,000 individuals a manageable number, but the feral Mustang population currently exceeds 33,000. More than half of all Mustangs in North America are found in Nevada (which features the horses on its State Quarter in commemoration of this), with other significant populations in Montana, Wyoming and Oregon. Another 30,000 horses are in holding facilities.