Horses were native to North America - until humans arrived. It's probable/possible that humans killed off the North American horses and drastically altered the ecosystem that existed when they arrived by hunting, burning, killing off large predators, etc. It's just as probable/possible that the end of the Ice Ages altered the ecosystem resulting in the extinctions of horses and megafauna.
We don't know which was responsible for the disappearance of horses in North America.
It's also clear that killing off the feral horses (they aren't wild, they are the offspring of released/escaped animals) is not politically possible even if it were desirable.
As with the other examples of species that are now in our ecosystems and cannot or will not be removed, we need to learn how to restore a balance. The specific ones mentioned - zebra mussels, gobies, kudzu, fire ants, iguanas, rats, snakes, and goats - may be more destructive or annoying than many others, but maybe we should spend more time and money into researching how to balance their impact than on how to simply kill them off.
Horses disappeared from North America maybe 12,000 after evolving here for millions of years. They were re-introduced 600 years ago. What period of time do we allow for claiming a species is "native"?
Of interest:
According to these results, it appears the genus Equus evolved from a Dinohippus-like ancestor ~4-7 mya. It rapidly spread into the Old World and there diversified into the various species of asses and zebras. A North American lineage of the subgenus E. (Equus) evolved into the New World stilt-legged horse (NWSLH). Subsequently, populations of this species entered South America as part of the Great American Interchange shortly after the formation of the Isthmus of Panama, and evolved into the form currently referred to as "Hippidion" ~2.5 million years ago. Hippidion is thus unrelated to the morphologically similar Pliohippus, which presumably went extinct during the Miocene. Both the NWSLH and Hippidium show adaptations to dry, barren ground, whereas the shortened legs of Hippidion may have been a response to sloped terrain.[27] In contrast, the geographic origin of the closely related modern E. ferus is not resolved. However, genetic results on extant and fossil material of Pleistocene age indicate two clades, potentially subspecies, one of which had a holarctic distribution spanning from Europe through Asia and across North America and would become the founding stock of the modern domesticated horse.[28][29] The other population appears to have been restricted to North America. One or more North American populations of E. ferus entered South America ~1.0-1.5 million years ago, leading to the forms currently known as "E. (Amerhippus)", which represent an extinct geographic variant or race of E. ferus, however.
Genome sequencing
In June 2013, a group of researchers announced that they had sequenced the DNA of a 560–780 thousand year old horse, using material extracted from a leg bone found buried in permafrost in Canada's Yukon territory.[30] Prior to this publication, the oldest nuclear genome that had been successfully sequenced was dated at 110–130 thousand years ago. For comparison, the researchers also sequenced the genomes of a 43,000 year old Pleistocene horse, a Przewalski's horse, five modern horse breeds, and a donkey.[31] Analysis of differences between these genomes indicated that the last common ancestor of modern horses, donkeys, and zebras existed 4 to 4.5 million years ago.[30] The results also indicated that Przewalski’s horse diverged from other modern types of horse about 43,000 years ago, and had never in its evolutionary history been domesticated.
http://en.wikipedia.org/wiki/Evolution_of_the_horse#Modern_horsesHorses have long since reached a certain balance in the Western ecosystem. Removal of predators and commercial use of grazing areas have made more impact than the horses themselves, IMO.
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