There is really only one species of horse (Equus ferus), also called the Wild Horse. But there are three sub-species recognized. The common domestic horse is a subspecies (Equus ferus caballus). The other two are the "truly wild" horse sub-species, the Tarpan (Equus ferus ferus), now historically extinct, and the Przewalski's Horse (Equus ferus przewalskii), named after the Russian explorer Nikolai Przhevalsky. It is a rare Asian animal, also known as the Mongolian Wild Horse. The Tarpan, or European Wild Horse, was found in Europe and much of Asia. It survived into the historical era, but became extinct in 1909, when the last captive died in a Russian zoo.
But even the above classification is debated. Some sources state that horses have been assigned to two different species, E. caballus (including ferus and gmelini) and E. przewalskii, but recent authors include przewalskii in caballus.
(Sources: Wikipedia and www.ncbi.nlm.nih.gov/Taxonomy/Browser)
