It is responsible for the production and regulation of hormones. While the tumour itself is benign, the cells within the tumour produce excess hormones, creating an imbalance in the horse's body. The cause of the tumour itself is usually not known.Cushing's disease is most common in horses over 20 years old, although it has been known to occur in horses as young as seven year old. It is equally prevalent in both genders and is found more often in Morgans than in any other breed. Ponies may also have a slightly higher predisposition.The most notable symptom of Cushing's disease is the growth of a long, shaggy, coat of hair. This irregularly long and sometimes coarse wavy hair often persists throughout summer months and is a critical signal that a horse may have Cushing's disease. This may be accompanied by sweating and flaking of skin.