In the case of the fetlock, the capsule extends out of the back of the actual joint and forms a pouch above the sesamoid bones behind the cannon bone. If this pouch of the joint capsule becomes thickened or distended, the resulting swelling on the back of the fetlock is known as an articular windgall. imagined as a sausage of fluid which envelopes the tendons as they run over the back of the fetlock.