A mustang is a type of wild horse. Most mustangs roam free in the western part of the United States.
It's actually more accurate to call mustangs feral horses, since they live like wild animals but are directly descended from horses that were domesticated. These ancestors were Spanish horses brought to North America by explorers during the 1500s. The cowboys who caught and sold mustangs during the 18th and 19 centuries were known as mustangers. The word mustang comes from the Spanish mestengo, "wild, stray, or having no master."