Coon foot is otherwise known as "Degenerative Suspensory Ligament Desmitis" and is most often seen in gaited horses, especially Peruvian Pasos and Paso Finos, but sometimes occurs in other breeds.
It's also called Passive Suspensory Ligament Failure and Deep Suspensory Ligament Desmitis.
It affects both front legs and sometimes the hind legs as well. The suspensory ligament breaks down under normal weight bearing (In Pasos it's been seen in yearlings), causing the fetlock to drop and joint problems in the pastern. DSLD appears to be hereditary, especially in Pasos (which often have to be euthanized and at best are pasture sound only) - so an animal with DSLD should not be bred (in any case, a mare with coon foot is likely to suffer pain from carrying the extra weight of the foal).
It can sometimes be treated with special shoeing and trimming.