Hereditary equine regional dermal asthenia (HERDA), also known as hyperelastosis cutis (HC), is an inherited autosomal recessive connective tissue disorder. It develops from a homozygous recessive mutation that weakens collagen fibers that allow the skin of the animal to stay connected to the rest of the animal. Affected horses have extremely fragile skin that tears easily and exhibits impaired healing. In horses with HC, the skin separates between the deep and superficial dermis. There is no cure. Most affected individuals receive an injury they cannot heal, and are euthanized. Managed breeding strategy is currently the only option for reducing the incidence of the disease.

The disease is found primarily in the American Quarter Horse, specifically in cutting horse lines. Affected horses have been found to trace to the stallion Poco Bueno, or possibly, farther back to one of his ancestors. Researchers have now named four deceased Quarter Horse stallions that were carriers and produced at least one affected HERDA foal; they are Dry Doc, Doc O'Lena, Great Pine, and Zippo Pine Bar. These stallions all trace to Poco Bueno through his son and daughter Poco Pine and Poco Lena. Other breeds affected are the American Paint Horse, and the Appaloosa and any other breed registry that allows outcrossing to AQHA horses.

HERDA is characterized by abnormal skin along the back that tears or rips easily and heals into disfiguring scars. The skin is loose, and hyper-elastic in affected horses. This collagen based abnormality affects their heart valves too and their mechanical properties are found to be inferior to native horses. Symptoms typically don’t appear until the horse is subjected to pressure or injury on their back, neck or hips, usually around two years of age. However foals can show signs when injured, while other horses mature and only show signs in the joints. The expression of HERDA is variable, and the phenotypic range of expression is still being determined.

After being tested, the results either show "normal", "carrier" or "affected" and can be determined by DNA testing of either a blood or hair sample.

• N/N — Normal: does not have the HERDA gene
• N/HRD- Carrier: carries one copy of the gene
• HRD/HRD- Affected: has two copies of the gene

The expected lifespan of an affected horse is 2–4 years. There is currently no cure for this disease. To prevent it from occurring, the only solution is not to breed horses who both carry the HERDA allele.