On the East Coast, there are a couple of barrier islands that have and auction off, wild horses. The story on them is that Spanish Conquistadors brought them over to Mexico and later up through florida, ect... and they made it out to the Coast of VA. I don't know about the western horses, but have read a little about the mastadons and other animals coming over the Asian landbridge of Alaska. My guess is they came up from Mexico, though, without google's assistance

Oh, I forgot, if you have a achance, the Bureau of Land Management website has all that stuff on there, as they are the ones that round up themustangs and auction them. I ijust don't remember what it says.