That used to be the belief, dunno if it's still generally believed. Used to be the wolf was thought to be from the Americas, and spread into Asia, etc, giving us the domesticated dog after some time. :') Camels also existed in the Americas (fossil record) and went extinct. When the sea level was hundreds of feet lower (which happened a number of times), the continental shelf was exposed, and I think the continental shelf had it going on... :')
Nice page about the "Heavenly Horse" BTW.
I think that's well established. I've dug for fossils of an extinct miniature horse, Plesippus shoshonensis gidley, in the Hagerman Fossil Beds in southwest Idaho. The Hagerman Horse has the distinction of being the earliest record of Equus, the genus that includes all modern horses, donkeys, and zebras.
It's odd to find this thread right now. I'm re-reading DeVoto's edition of Lewis & Clark's journals and got to wondering how the Nez Perce became master horse-breeders so quickly after the horse's reintroduction to America. The Appaloosa breed (aka Palouse War Horse) was legendary in Idaho where I grew up.
It was interesting to read Lewis & Clark's praise about them. I agree there doesn't seem to be any hard evidence as yet that any such horses preceded Columbus. But I think there's some logic to the argument they came from an area where pre-Columbian contacts with Asia, if any, were likely to have occurred. But one has to ask: why the Nez Perce, a mountain tribe, and not aboriginals closer to the Pacific coast?