Fetlock is the common name for the metacarpophalangeal and metatarsophalangealjoints (MCPJ and MTPJ) of horses, large animals, and sometimes dogs. It is formed by the junction of the third metacarpal (forelimb) or metatarsal (hindlimb) bones (common name: the cannon bones) proximad and the proximal phalanx distad (common name: the pastern bone). Paired proximal sesamoid bones articulate with the palmar or plantar distal surface of the third metacarpal or metatarsal bones and are rigidly fixed to the proximo-palmar / -plantar edge of the proximal phalanx.
While sometimes the fetlock is colloquially referred to as an "ankle," even by horse experts, that terminology is not correct. The fetlock actually is a metacarpophalangeal joint which corresponds to the human upper knuckle, such as that on the ball of the foot.