Although horses were once native to North America, no horses were found here by the time the Europeans arrived. Spanish explorers "reintroduced" horses into the American West, and many of these horses escaped or were released to form wild (or feral) herds.
Today, these wild horses descended from Spanish stock are known as Mustangs, and they come in almost every possible coat color and pattern. Natural selection has given these wild horses a very hardy constitution and a tough, independent nature. They are usually smaller than average (13 to 15 hands), have thick necks, and can have luxuriant manes and tails (a relic of their Spanish and Andalusian ancestry).
I spotted these wild horses roaming the foothills of Arizona's Chiricahua Mountains in March of 2006.
I don't know if they were purely wild horses or just ranch ponies that had either been turned loose or had escaped, but they were roaming free and seemed quite wary of me.
This luxuriantly-maned chestnut was always out in front.
While this whitish gray (this is not the same one as in the top photo) always took up the rear.
To control wild horse populations and prevent them from competing with cattle for forage on public rangelands, the Bureau of Land Management (BLM) has a program to capture wild free-roaming horses or burros found on public rangelands and place them up for adoption. Sadly, not all of these horses and burros will find homes.