The word "fetlock" sometimes called the "horses ankle" originally referred to the tuft of hair found on the rear of the joint between the cannon bone and the pastern. The fetlock is in fact a hinged joint, which allows flexion and extension, but minimal rotation. This portion of the anatomy of horses, large animals and even dogs is wrongly referred to as an ankle, it does in fact, correspond to the human upper knuckle, such as that found on the ball of the foot.

The Fetlock is prone to injury, particularly in Thoroughbred Race Horses accounting for almost 50% of their catastrophic racing injuries.

More Info: en.wikipedia.org