Hereditary Equine Regional Dermal Asthenia (HERDA) is a degenerative skin disease that primarily affects the American Quarter Horse. Loose skin is often an early indication of the disease, and severe seromas, hematomas, ulcerations usually develop around 1.5 years of age. There is no cure, and the majority of affected animals have to be euthanized within 2-4 years.

Hoof Wall Separation Disease (HWSD) is characterized by separation and breakage of the dorsal hoof wall extending up from the sole. Symptoms of affected ponies begin at less than 6 months of age, and often lead to development of chronic laminitis.

Hydrocephalus (HDC) is an abnormal build up of cerebral spinal fluid around the brain. It is believed that a narrowed passage within the brain prevents normal fluid absorption, leading to an obvious external cranial distension. Affected foals are often stillborn and are associated with dystocia in the dams.

Hyperkalemic Periodic Paralysis (HYPP) is caused by defects in sodium channels within the muscle, which can lead to paralysis, collapse and sudden death. Affected animals experience episodes of muscular weakness or tremors.