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.