While paleontological evidence suggests that horses were present in North America as recently as 10,000 years ago, they disappeared with the arrival of humans (part of the Great Columbian Extinction Event). Horses were reintroduced to North America by the Spanish. Repeated use by Spanish, French, English, American, Mexican and Native travellers inevitably resulted in escaped horses, which have reestablished as feral populations throughout the American West.
Today Nevada has the largest population of wild horses in the United States, numbering more than 26000 since 1988. Current numbers are heavily politicized and debated. Many ranchers of the area consider them pests who would compete with forage for their livestock and spread diseases, and who frequently shot and trapped them, while animal lovers see the feral populations as a successful reintroduction and should be left alone. In 1971 Congress passed The Wild Free-Roaming Horse and Burro Act, which declared these animals to be "living symbols of the historic and pioneer spirit of the West." The horse populations are to be considered along with other resources in the lands held by the Bureau of Land Management (BLM). Currently the BLM "manages" the herds, staging periodic roundups by helicopter when it feels the herd populations are too high (which seems to be "always"), and training the horses up for adoption. This remains a very controversial issue in the State of Nevada, ranching, environmental protection, and animal rights circles.
Here a stallion (grey) is leading a small herd through the streets of Virginia City. Perhaps like the Bighorn Sheep near Boulder City the drought has drastically reduced forage.
Virginia City, Nevada