Answer:
Bulls that are born to buck. The only way you can tell if they're good bucking bulls is if you put them in a round-robin rodeo to test not only the riders, but the bucking ability of the bulls as well. Pretty well any breed of bull can be a good bronc, be it a Texas Longhorn, a Brahman, a Charolais, or even a Hereford. It just pays to have him tested out in the ring first before really confirming is abilities.