Horses have four fetlocks. They are located between the hoof and knee in the front legs and hoof and hock in the hind legs. The fetlock joint is where the cannon bone meet the long pastern and they have two little seasmoid bones at the back of them. Whilst they are in the same location as our ankles i.e. the joint between knees and feet they are actually designed like our wrists. These hinge joints are designed to move forwards and backwards and have a restricted circular or sideways movement.
Most horses will have no extra hair beyond their normal coat on their fetlocks however some have long thick hair, feathers' protruding from the rear of the leg. Hairy fetlocks are a good place for harvest mites to hide. These can be detected by the horse stamping vigorously on the ground or in the horse's absence, by bare scrapes in the ground where the turf has been rolled out by constant stamping. If found earlier enough you can roll these little bits of turf back into place without any lasting damage. (Not of course, that I'm admitting 'we' suffer from harvest mites!)
At the very rear point of the fetlock is a thick lump of scaly skin, like that found on a horse's chestnuts. These are known as ergots and seem to serve little purpose in the horse's life beyond ripping open the skin of unsuspecting grooms and farriers.
Their most common complaint is that they can suffer from unsightly soft swellings. Temporary swellings which disburse with exercise can simply be an accumulation of fluid resulting from standing in the stable for too long (12 hours is all it takes for my heavyweight cob mare to start getting "Filled fetlocks".
Hot swellings accompanied by lameness and soreness usually indicate a sprain but can indicate osteoarthritis or chipped fetlocks. Unbeknown to me my old pony had a chipped fetlock. It didn't cause him any trouble until I let him become overweight and lent him to a young lady who didn't appreciate the dangers of jumping on hard ground. Then he went absolutely and totally hopping lame! The poor little fellow was on box rest for weeks after that to help 'quieten down' the floating bone chip.
Cool unsightly swellings which don't go away after exercise, aren't accompanied by lameness and can be manipulated up and down or from side to side like little bags of water are usually Windgalls. These are simply low grade injuries, usually caused by too much concussion from exercising on hard ground which once gained are there for life and shouldn't be a problem for the horse. Horses that are 'upright' in the pastern are particular prone to windgalls.
Fetlocks can be protected during exercise with Fetlock boots, which are made of leather to protect the joints from self-harm by brushing the shoe of one leg against the joint of the other leg.
Due to the number of tendons running over or attached to fetlock bones there is a high risk of ligament/tendon injury. Horses with severe untreated tendon troubles can be seen standing with their fetlocks sunk so far down that they are almost touching the ground. Another problem they are prone to is being cut from by wire fences after high-spirited youngster have had a race around their field and mis-time their halts resulting in spectacular slides under the fence!