Mongolian wild horse
The Mongolian wild horse (Equus przewalskii Poljakow) is the only wild horse recognized by all zoologists as a pure wild form in the zoological sense. This has led to the theory that is the ancestor to all our domestic horses. This theory is disseminated as fact in most books and articles today. However, Japanese geneticists have documented in 1995 through mtDNA analyses that Mongolian wild horse, or Przewalsi's horse, is NOT an ancestor of our domestic horses!
This issue is being made complicated by the fact that, because Przewalski's horse is the only wild horse scientifically described and documented, and it belongs - together with all other horses - to the same species, its zoological name must apply to all of today's horses according to zoological nomenclature systematics.
So, if it is said that all horses stem from Equus przewalskii, that is correct and wrong at the same time: if it is supposed to mean they stem from Przewalski's horse, i.e. the Mongolian wild horse, it is wrong; if it is meant to say that whatever wild ancestors there were of our domestic stock, they would all belong to the species Equus przewalskii, it would be correct. The Mongolian wild horse, or Przewalski's horse, stems from the same root as all other horses we know, but it does not represent this root!
Some zoologists and paleo-zoologists think that there were several forms of wild horses that our domestic horses derived from. One such form is still around in the British Exmoor pony, another one in the Sorraia horse. It would be foolish to claim the Sorraia horse to be pure anymore, but its status is most likely that of a direct descendant of an ancestral form, the closest thing we have left to that form.
The Mongolian wild horse survived in zoos all over the world. It, too, isn't pure anymore, but it is still a good representative of the original subspecies.