The cushionlike projection, bearing a tuft of long hair, on the back side of the leg above the hoof of the horse and similar animals. Also, the joint of the limb at this point (between the great pastern bone and the metacarpus), or the tuft of hair.
The "ankle-like" joint between the cannon bone and the first bone of the foot (P1). Although it's called the ankle of the horse because of its outward appearance, the fetlock corresponds to your middle knuckle (front fetlocks) or the middle toe at its attachment to the foot (rear fetlocks).
Fetlock is the common name for the metacarpophalangeal and metatarsophalangeal joints (MCPJ and MTPJ) of the horse. 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.