The metacarpophalangeal joints (MCP) are situated between the metacarpal bones and the proximal phalanges of the fingers.

[1] Being condyloid, they allow the movements of flexion, extension, abduction, adduction and circumduction (see anatomical terms of motion) at the joint.

In many quadrupeds, particularly horses and other larger animals, the metacarpophalangeal joint is referred to as the "fetlock".

In fact, although the term fetlock does not specifically apply to other species' metacarpophalangeal joints (for instance, humans), the "second" or "mid-finger" knuckle of the human hand does anatomically correspond to the fetlock on larger quadrupeds.

This article incorporates text in the public domain from page 332 of the 20th edition of Gray's Anatomy (1918)