From what animal is the domestic horse descended?

Two horses
   As far as we know, the domestic horses of today have descended from at least three different wild-horse stocks. The now-extinct tarpan, or a closely related horse, seems to have been the ancestor of the modern Arab horse — the domestic animal of North Africa. The tarpan was a light-boned, small animal, standing four feet at the shoulders; its forehead was broad and its mane stiff. It roamed widely, long ago, over Europe and Eurasia.
   The ancestor of our draft horses was a stronger and larger animal inhabiting western Europe. The Mongolian pony, or Przewalski's horse, of central Asia, represented of the wild-horse stocks that gave rise to modern stocks. It is still found in Mongolia. It is about four and a half feet high, is light brown and has a blackish tail and a black, stiff mane.