Wild horses are icons of the West, but growing herds have become a costly problem. Tribes say the horses damage their land and need to be managed — maybe by bringing back slaughterhouses.WARM SPRINGS INDIAN RESERVATION —Here on this reservation in north-central Oregon, horses are woven deeply into daily life. They are traditionally used by tribal members in their work and their culture, whether it be for rodeos or horse parades. Gathering, breaking and selling wild horses has long been part of the tribe's economy. Horses that don't make the grade are sold for slaughter. But the nation's final three...