buck off
Of a horse, to stand on its back legs, in an attempt to knock a rider off its back. A noun or pronoun can be used between "buck" and "off." Be careful with that horse—he's pretty ornery and might try to buck you off.
buck someone off
[for a horse or similar animal] to rear up in an attempt to shake off its rider. The horse tried to buck Sharon off, but she held on tight. The horse bucked off its rider.