The wild horse is a species of the genus Equus, which includes as subspecies the modern domesticated horse (Equus ferus caballus) as well as the undomesticated tarpan (Equus ferus ferus), now extinct, and the endangered Przewalski’s horse (Equus ferus przewalskii). Przewalski’s horse was saved from the brink of extinction and reintroduced successfully to the wild. The tarpan became extinct in the 19th century, though it was a possible ancestor of the domestic horse, and roamed the steppes of Eurasia at the time of domestication.
“Blame it or praise it, there is no denying the wild horse in us.”
― Virginia Woolf, Jacob’s Room
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, resulting in horse breeds such as the Konik and 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 to refer 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 subspecies (Equus ferus caballus), and should not be confused with the two truly “wild” horse subspecies extant into modern times.