Of Horses and Humans

The end of the Ice Age saw the extinction of the majority of large mammals native to North America. Mammoths, ground sloths, saber-toothed cats, giant armadillos called glyptodonts, and short-faced bears were among the species that were killed off, most likely by humans. So were horses. The horses of prehistoric North America, admittedly, were of a different species than today's domestic horse, but resembled them in many respects.
After this, North America would be without horses until European settlers brought domestic horses--which are descended from the Przewalski's horse of central Asia--in the 1500s. The feral descendants of these horses are the "wild" horses that now populate the North American continent.
This has caused a considerable debate on both sides of the ecological spectrum. There are people who believe that feral horses, such as the Chincoteague Ponies of Virginia and Maryland the mustangs of the southwest, essentially fill the niche of the extinct North American horses and should therefore be considered native. Then there are people who claim that any non-native animal, no matter how closely related it is to a formerly native one, should be considered invasive.
The problem, of course, is that horses are immensely popular animals. Other feral livestock, such as pigs, are much more readily assumed to be dangerous and invasive, but few people see horses as an ecological hazard.