The mule is a hybrid sired by a male ass (Equus asinus) out of a female horse (E. caballus). The opposite cross, very seldom made, produces the hinny (a hybrid between a stallion and a female ass). The mule and the hinny are usually sterile, but there have been two authenticated cases of mules producing living progeny. Male mules, often called horse mules, are almost always castrated to make them more tractable as work animals.