The word bronco is derived from the Spanish word broncos, meaning rough, and is used to describe a type of horse, rather than a species or a breed. American cowboys used the term to refer to untrained or partially trained horses.
The word bronco can also refer to a horse that frequently bucks. Bucking is natural behavior for a horse and it is believed that it is a defense against predatory animals. Horses that buck frequently are sold and trained for rodeos. There is only one species of wild horse in the world, and it lives in Mongolia; wild horses in the United States are domestic horses that have become feral.