Buck is the movement in which a horse drops its head toward the ground, rounds and arches its back, and kicks out with both hindlegs simultaneously or in rapid succession, projecting the hindquarters upward. The action is a natural equine behavior, horses in open pasture buck during play, at the start of a fast canter, or when energized by cold weather. Under a rider it is dangerous: a bucking horse can unseat its rider and, if the bucking is severe, cause a fall.
Bucking during ridden work carries clinical significance. A horse that bucks consistently at the canter transition, when leg is applied, or on a particular rein is often communicating pain rather than resistance. Back pain, ill-fitting saddle, hock soreness, and gastric ulcers are common physical causes. A horse that has bucked successfully in response to an uncomfortable stimulus has been reinforced to repeat the behavior, which is why distinguishing pain-driven bucking from learned behavior requires a veterinary workup before a training intervention.
In rodeo sport, bucking is a formalized and scored discipline. Rodeo broncs, both saddle bronc and bareback, are selected and bred for their athletic bucking ability, and the sport scores both the animal’s performance and the rider’s. In cutting and reining, a horse that bucks unexpectedly is disqualified. The term also appears in the phrase “bucking out”, a horse blowing off excess energy with a short burst of bucks before settling into work, which is distinct from sustained or violent bucking. Understanding what precedes a buck is the diagnostic starting point for addressing it.
Further Reading: The Wikipedia article on bucking in horses covers the behavior’s natural context and rodeo applications; the Merck Veterinary Manual’s section on lameness in horses is the clinical starting point for diagnosing pain-driven bucking.