A hunter's bump is technically known as sacrcoiliac subluxation. It is a tearing of the fibrous attachment that holds the pelvis and spine of the horse together. Once the attachment is loosened, the pelvis of the horse actually shifts out of place causing the characteristic "bump" on the horse's rump that you are referring to. The bump is actually the bony prominence of the pelvis which becomes more visible after this subluxation happens.
This injury is most common in jumping horses, carriage horses, etc. and is usually the result of repeated trauma rather than a single accident. When this condition is noticed and diagnosed early, the prognosis is favorable. With repeated injury or extensive tearing of the ligament, permanent lameness may occur. If your horse has the classic hunter's bump and is currently sound, the injury may have taken place some time ago and has healed sufficiently to reestablish a connection between the pelvis and spine. Even in horses that have healed from this condition, owners should take precautions such as thorough warm-ups and cool-downs, avoiding slippery footing, etc. so as to not reinjure the horse.
A hunter's bump classification of blemish versus unsoundness depends. Technically, it is an unsoundness since its occurrence causes a functional problem in the horse. However, if a horse is sound by all examinations and the hunter's bump is the remainder of an old sacrcoiliac subluxation which causes no changes to the gait and performance of your horse, then you may classify it as a blemish (since it is a defect that does not interfere with the horse's action, and it may diminish value but not function).
