Horses typically lose all of their deciduous teeth- those they get as babies- somewhere between one and four years old. When one of these teeth fails to shed, it is called a "dental cap", or a "retained dental cap". This can cause pain in a horse's mouth, which will lead to difficulty eating.
If you suspect your horse has a retained dental cap, it is important to seek immediate veterinary care. A veterinarian is able to remove the cap, and the horse should recover with no long-term effects.