Bucking is a movement performed by an animal in which it lowers its head and raises its hindquarters into the air while kicking out with the hind legs. It is most commonly seen in herbivores such as equines, cattle, deer, goats, and sheep. Most research on this behavior has been directed towards horses and cattle.
A rodeo horse, bucking.
A loose horse may buck due to aggression or fear, as the very high kick of this horse suggests
Bucking is a normal behavior for a horse with an overabundance of energy, and in a loose horse, may simply be playful behavior, as here
Horse bucking as an act of disobedience or discomfort
A bucking bull is a bull used in rodeo bull riding competition. They are usually a Brahman crossed with another breed, weighing 1,500 pounds or more, selected for their tendency to "leap, plunge and spin" when a human is on its back. Circa mid-20th century breeders began selecting bulls for bad temperament, that would buck when ridden.
Many of the best bucking bulls trace their lineage to bulls owned by Charlie Plummer of Oklahoma. These are known as Plummer bulls.
Modern-day bucking bull
Mildred Douglas riding a bucking bull c. 1917