Horse outward similarity is an unreliable measure of relatedness. Besides the horse, there are six other species of genus "Equus" in the Equidae family. These are the ass or donkey, "Equus asinus"; the mountain zebra, "Equus zebra"; plains zebra, "Equus quagga"; Grévy's zebra, "Equus grevyi"; the kiang, "Equus kiang"; and the onager, "Equus hemionus". Horses can crossbreed with other members of their genus. The most common hybrid is the mule, a cross between a "jack" (male donkey) and a mare. A related hybrid, a hinny, is a cross between a stallion and a jenny (female donkey). Other hybrids include the zorse,
Equus (play) Equus (play) Equus is a play by Peter Shaffer written in 1973, telling the story of a psychiatrist who attempts to treat a young man who has a pathological religious fascination with horses. Shaffer was inspired to write "Equus" when he heard of a crime involving a 17-year-old who blinded six horses in a small town near Suffolk. He set out to construct a fictional account of what might have caused the incident, without knowing any of the details of the crime. The play's action is something of a detective story, involving the attempts of the child psychiatrist Dr. Martin