A bronc is a bronco: a horse that is still wild, or that bucks hard and tries to throw the rider off. The short word is what cowboys, ranch hands, and rodeo people actually say; bronco is the fuller, more dictionary-looking form. You hear it in bronc riding — staying on a horse that does not want you there — and in bronc buster, a person whose job is to tame that kind of horse. It is informal Western American English, not a riding-school word. Though actually, a bronc in a modern rodeo is often a horse trained to buck, not a wild animal just in from the plains; the old picture and the sport have drifted a little. Same horse as a bronco; just the shorter name.

野馬（口語，bronco 的簡稱；牛仔、競技騎術用語）