Anatomically, the fetlock joint is not really the horse's ankle at all. It is the metacarpo-phalangeal joint of the forelimb, the equivalent of your middle (upper) knuckle joint. (The hind limb fetlock is the equivalent of the middle metatarso-phalangeal joint at the front of your foot.)

The fetlock is an extremely dynamic, sensitive, high-motion joint; a critical component of the intricate mechanism of the lower limb of the horse.

The normal fetlock is supported in a sling made up of the flexor tendons and the suspensory ligament on the rear of the joint. In a normal healthy horse, large extensor muscles on the front of the upper limbs transfer their force via long extensor tendons to the front of the lower limb bones below the fetlock, flipping it forward.

There are thus several ways in which the fetlock can fold forward when the horse tries to bear weight. If the extensor muscles or tendons are cut or don't function, the fetlock can roll forward or knuckle over. This can be caused by a wound that severs the tendon. Nerve or spinal dysfunction can cause similar signs because of loss of extensor muscle function.

The second way in which the fetlock will fold over is if the flexor tendons on the back of the limb are short enough to prevent the fetlock sling function and force it upright and over. This happens in young growing horses with severely contracted tendons. We sometimes also see this over time in chronically lame limbs, in which normal weight bearing does not take place, and the tendons and joint capsule contracts up to the point that the fetlock no longer functions.