It is called an ergot (Definition: The ergot is thought to be another vestige of a toe similar to the chestnut. It grows from the rear underside of the fetlock joint. The composition is somewhat like soft hoof wall material. It tends to be somewhat tougher than the chestnut and not so easy to peel away with your fingers. On some horses it will be a very small nub, while on others it may be much larger, curling and splitting as it grows. If it becomes long and unsightly you can trim it down with hoof clippers or ask your farrier to trim it when he trims your horse’s feet). My horse has these as wekk. I will be trimming them this weekend.