North America was the original home of the horse species. Horses evolved here, and equine species thrived here for over 57 million years. The plant and animal communities of North American ecology evolved with horses playing an integral role.
About 8,000 - 10,000 years ago, for reasons not yet fully understood (meteors, climate change, disease pandemic, and human hunting pressures are among the possibilities), horses are believed* to have become extinct in the land of their origin. Luckily by that time they had migrated to Asia, where they spread into Europe and North Africa.
(* see "Alternative Histories")
During the Pleistocene Era, there were more than 50 species of equids in the Americas.