A true single-toed horse, Pliohippus roamed the grassy plains of North America about 12 million years ago, and may have survived into the Pliocene epoch. Although it closely resembled modern horses, there's some debate about whether the depressions in its skull, in front of its eyes, are evidence of a parallel branch in equine evolution.
Merychippus was endemic to North America during the Miocene from 20.43—10.3 Ma living for approximately 10.13 million years. It had three toes on each foot and is the first horse known to have grazed. Its name means "ruminant horse", but current evidence does not support Merychippus ruminating. Three lineages within Equidae are believed to be descended from the numerous varieties of Merychippus: Hipparion, Protohippus and Pliohippus. Image: Evolución del caballo VI a: El Merychippus