Native or Feral?
There has been significant controversy with regard to the "right" of the wild horse, particularly the mustang, to live freely on the ranges of the North American continent at present time. Unless new information is discovered that proves otherwise, it is accepted that mustangs are a "reintroduced" species. The American Heritage Science Dictionary, among most others, defines feral as "existing in a wild or untamed state, either naturally or having returned to such a state from domestication."
Wild Horses as Native North American Wildlife is an article by Jay F. Kirkpatrick, Ph.D. and Patricia M. Fazio, Ph.D. about the recent find that the modern or caballine horse, E. caballus, is genetically equivalent to E. lambei, a horse, according to fossil records, that represented the most recent Equus species in North America prior to extinction. Not only is E. caballus genetically equivalent to E. lambei, but no evidence exists for the origin of E. caballus anywhere except North America.
There is some evidence that suggests the horse never did become extinct in North America. The Pre-history of the Wild Horse, brings up some interesting issues and links to studies.