JEB (Junctional Epidermolysis Bullosa) is actually a horrible congenital deformity. It's also known as Red Foot Disease, Hairless Foal Syndrome or Epitheliogenesis Imperfecto (EI).
It's linked to two recessive genes, one of which is primarily found in Belgians and American Cream Drafts and the other in American Saddlebreds. The defect effects the production of a key protein that holds the skin on.
Affected foals are born normal, but rapidly develop hairless and then skinless lesions over points of wear, such as joints. In many cases the hoof is also shed. A key diagnostic feature is that the horse's front teeth are present at birth (which they should not be) along with oral ulcers.
The foals generally die or are euthanized within a week or so. And this disease has been recorded since the 1930s - it plagued draft horse breeders until a genetic test was developed to identify carriers. As it's a recessive gene, carriers are healthy.
(Incidentally, JEB also exists in humans, in two forms - Herlitz type, which is generally fatal, and non-Herlitz type, which is associated with a normal life span and appears to resolve itself after infancy, although long term damage to nails and teeth can be present).