The wild horse (Equus ferus) is a species of the genus Equus, which includes as subspecies the modern domesticated horse (Equus ferus caballus) as well as the endangered Przewalski's horse (Equus ferus przewalskii). The European wild horse known as the tarpan that went extinct in the late 1800s has previously been classified as a subspecies of wild horse (Equus ferus ferus), but more recent studies have cast doubt on whether those horses were truly wild or if they actually were feral horses or hybrids.

Przewalski's horse had reached the brink of extinction but was reintroduced successfully into the wild. The tarpan became extinct in the 19th century but is theorized to have been present on the steppes of Eurasia at the time of domestication. However, other subspecies of Equus ferus may have existed and could have been the stock from which domesticated horses are descended. Since the extinction of the tarpan, attempts have been made to reconstruct its phenotype using domestic horses, resulting in horse breeds such as the Heck horse. However, the genetic makeup and foundation bloodstock of those breeds is substantially derived from domesticated horses, so these breeds possess domesticated traits.

The term "wild horse" is also used colloquially in reference to free-roaming herds of feral horses, for example the mustang in the United States, and the brumby in Australia. These feral horses are untamed members of the domestic horse subspecies (Equus ferus caballus), not to be confused with the truly "wild" horse subspecies extant into modern times.

Evidence supports E. ferus as having evolved in North America about 1.1 - 1.2 million years ago. Around 800,000 - 900,000 years ago, E. ferus migrated west to Eurasia via the Bering Land Bridge, and south to South America via the Isthmus of Panama as part of the Great American Interchange. By the mid-late Pleistocene, it had an extremely large range across the Americas, Eurasia, and North Africa, across which it was abundant. There have been several fossil horse taxa from throughout this range, such as Equus lambei and Amerhippus, that were formerly considered distinct species, but genetic and morphological analysis supports them as being conspecific with E. ferus.

By the latest Pleistocene or early Holocene, American populations had disappeared as part of the Quaternary extinction event, leaving only the Old World populations. It remained widespread there and was ultimately also domesticated around 3600 B.C., but wild populations continued to decline. The last completely wild populations of the tarpan went extinct in Eastern Europe and the southern parts of Russia around the late 19th century, and the Przewalski's horse of Central Asia became extinct in the wild in 1969. However, over the past few centuries feral horses have been introduced to all continents except Antarctica, and Przewalski's horses have been reintroduced to their former habitats in Mongolia.