Bronco, or bronc, is a term used in the United States, northern Mexico and Canada to refer to an untrained horse or one that habitually bucks. It may refer to a feral horse that has lived in the wild its entire life, but is also used to refer to domestic horses not yet fully trained to saddle, and hence prone to unpredictable behavior, particularly bucking. The term also refers to bucking horses used in rodeo "rough stock" events, such as bareback bronc riding and saddle bronc riding. The silhouette of a cowboy on a bucking bronco is the official symbol for the State of Wyoming.
In modern usage, the word is seldom used any longer to refer to a "wild," or more accurately, a feral horse, because today, the modern rodeo bucking horse is a domestic animal. Some are specifically bred for bucking ability and raised for the rodeo, while others are spoiled riding horses who have learned to quickly and effectively throw off riders. Informally, the term is often applied in a joking manner to describe any horse that acts up and bucks with or without a rider. The Wild Horse Protection Act prevents the capture of mustangs from the wild for commercial use, and though the law has been weakened in recent years, "wild" mustangs and other completely untamed horses are still no longer used on the rodeo circuit, as bigger, more powerful animals that are sufficiently domesticated to be handled from the ground for veterinary care, travel, and stabling in small pens are more desirable as rodeo stock.
The term comes from the Spanish language word bronco, meaning "rough", which in Mexican usage also describes a horse. It was then borrowed and adapted in US cowboy lingo. It has also been spelled "broncho," though this form is virtually unknown in the western United States, where the word is most common. Many other instances of cowboy jargon were similarly borrowed from Mexican cowboys, including words such lariat, chaps, and buckaroo, which are in turn corruptions of the Spanish la riata, chaparajos, and vaquero.
In the early American west, most cattle ranches simply allowed young horses to grow up in a feral state on the open range, capturing them at maturity to be broken-in or "broke" to make them tame enough to ride. Sometimes mustangs were rounded up as well, as the two populations often mixed.