Bull Hunter by Max Brand

Diablo greeted them with a throwing up of his formidable head. He took his place in the very middle of his corral, but when Bull Hunter and his small guide reached the bars, the black stallion seemed to go suddenly mad. He flung himself into the air and came down bucking. Back and forth across the corral he threw himself in the wildest swirl of pitching that Bull Hunter had ever seen or ever dreamed of.

"He's an educated bucker, you see? " said the boy in admiration. "They ain't any trick that he don't know. Look!"