Modern horses are believed to be descendants of those introduced by Spanish settlers in North America during the 16th century. Horses naively roamed North America until approximately 10,000 years ago, when they became extinct. The cause of this extinction is unknown, although climate change and hunting are probable factors.
Eohippus, or the "dawn horse," is believed to be one of the first ancestors of horses. It is characterized by its diminutive stature and toes that each end in a hoof-like structure. The Eohippus has origins that date back over 1 million years ago. Modern horses, in comparison, have one hoofed toe on each foot and vestigial toes that resemble bumps along the lower leg.