The evolution of the horse is fascinating! Equidae (the horse family) evolved over 55 million years, with more than 30 distinct species. The only genus surviving today is Equus, which includes horses, donkeys, and zebras.
Did you know that horses became extinct in both North and South America about 10,000 years ago? According to evidence from the North American fossil record, there were three waves of mass extinction ranging from the end of the Miocene to the late Pliocene.
Ten species of horses were lost during the first extinction event near the end of the Miocene (approx. 6 million years ago), and six more species become extinct during the early Pliocene (approx. 4.5 million years ago).
Three horse species survived the two earlier extinction events and of these, the two three-toed Hipparion species, the Nannippus and the Cormohipparion, faced the same fate about two million years ago in the late Pliocene.
In the very late Pliocene, about 2.5 million years ago, there were two to four species of equus present in North America. However, near the Pleistocene (approx. 11,000 years ago) there was another mass extinction event, which saw the loss of horses, mammoths, mastodons, ground sloths, camels and tapirs as well as many large carnivores (lions, saber-toothed tigers, and dire wolves).
While the exact cause of this most recent extinction event is still debated, the two commonly accepted theories are rapid climate change and overhunting by humans.
Horses were re-introduced to North America by the conquistadors beginning in 1493, when they brought Andalusians, Spanish Barbs, and the now extinct Spanish Jennets to the Caribbean Islands. I’ve written a bit about this previously.
American Museum of Natural History: Horse https://www.amnh.org/exhibitions/horse
Hulbert, Richard C. “The Ancestry of The Horse”. Horses Through Time edited by Sandra L. Olsen, Carnegie Institute, 1996, 11-43. (p. 28)