Dinohippus

An extinct ancestor of modern horses, Dinohippus emerged ~12 mya from late Merychippus species. It was a one-toed grazer and its skull, teeth and limbs were already very similar to our modern horse. It appears that the passive stay apparatus, which allows horses to stand without muscle power, first appeared in Dinohippus.
During the late Pliocene, Dinohippus was the most widely distributed horse in North America and it is assumed to be the direct ancestor of the genus Equus.