The hard part would be finding the right habitat for them where they will not encounter many humans.
Maybe you are aware of the vast region in Russia and Ukraine that was contaminated by radiation from the Chernobyl nuclear disaster. Humans are banned except for brief visits to study the ecosystem.
Animals and plants are thriving, and wild horses have been introduced there. Interestingly, the animals there, though radioactive, seem to be perfectly normal and healthy.
In reality, though, humans are a part of the ecosystem and thus affect the evolution of other species.
The earth is now undergoing massive extinctions of many species. This is not the first time, and previous mass extinctions were followed by the evolution of new species. The extinction of the dinosaurs led to the evolution of large mammals, for instance.
My idea is first take a mustang horse (since they are better adapted to the American environment) and mate it with the Przewalski's horse (as it closely resembles the extinct Equus lambei or American Wild Horse, is the only true wild horse left and is extremely hard to train since it's never been domesticated, which would make it harder for them to be taken out of the wild). Then once a breeding population is established, these horses should be introduced or in a sense re-introduced to a habitat with balanced with a significant amount of predators and more importantly far away from human agriculture, where they could be shot. Horses in America only went extinct about a couple thousand years ago (after the first humans arrived), so the niche still can be met. This is just an idea.
http://en.wikipedia.org/wiki/Przewalski's_horse
http://en.wikipedia.org/wiki/Mustang_horse
http://en.wikipedia.org/wiki/Yukon_Wild_Horse
