Horses are not considered part of the mega fauna. Mastodons, ground sloths, giant buffalo were not wiped out by humans.Well, yes and no. There is a fair bit of archaeological evidence that we almost hunted horses to extinction in Eurasia before we domesticated them. They were common game and likely easy and worthwhile to hunt. Lots of meat, there with no horns on top! We certainly hunted the Aurochs into oblivion as we morphed some of them into cattle. I have no trouble at all imagining that horses were hunted to extinction in the Americas as almost happened elsewhere. There are no horses left on this planet that do not contain the genetic markers of domesticates. Even the wild Eurasian ones are descended from an ancient domesticated breed. The North American Natives didn't domesticate horses until they were re-influenced by Europeans.