AKA foot scab, heel mites tail mange, leg mange, itchy leg, and scrotal mange.

Chorioptic mange is caused by an infestation of mites of the Genus Chorioptes. It occurs in cattle, horses, sheep, goats and rabbits. As you might have guessed from the alternative labels for this disease, it usually infects the feet, scrotum, thighs, and haunches. An infection results in broken, scabby skin and hair loss.

The mites are small (0.4 mm) whitish-grey parasites that live on (not in) the skin. They are sensitive to sunlight, and so usually live in long hair or crawl under the edges of scabs; chorioptic mange tends to be worse in the winter, when there is less sunlight and the animals spend more time indoors. The life cycle of these mites is not fully known, but it takes place entirely on the host animal. The mites spread by direct contact or through bedding. They cannot be transmitted from one species to another; each species of infected mammal has its own variety of mite.

The taxanomy of Genus Chorioptes is in a bit of a mess right now, but basically there are two species, each of which has a number of varieties.
Chorioptes bovis = Cattle, goats, horses, sheep and rabbits.
Chorioptes texanus = Reindeer, goats, and cattle.
Chorioptes equi = Sometimes used to refer to Chorioptes on horses.

Phylum Arthropoda
Class Arachnida
Order Parasitiformes
Suborder Astigmata