The wild horse (Equus ferus) is a species of the genus Equus, which includes as subspecies the undomesticated tarpan (Equus ferus ferus, now extinct) and the endangered Przewalski's horse (Equus ferus przewalskii). It is also thought to be the ancestor of the domestic horse (Equus caballus).

Przewalski's horse had reached the brink of extinction but was reintroduced successfully into the wild. The tarpan became extinct in the 19th century, though it is a possible ancestor of the domestic horse; it roamed 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 the domestic horse, 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.

Przewalski's horse has long been considered the only remaining extant, non-domesticated wild horse. A 2018 DNA study suggested that modern Przewalski's horses may descend from allegedly domesticated horses of the Botai culture. However, a 2021 study came to the conclusion that the Botai horses were wild Przewalski's horses and not domesticated.

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

 

Note:   The above text is excerpted from the Wikipedia article Wild horse, which has been released under the GNU Free Documentation License.