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 lambei horse, not an ass. The closest resemblance to "E." "lambei" is shown by the Mongolian horse, "E. caballus przewalskii", now believed to be extinct in the wild. In size and proportions, "E. lambei" resembles "E. caballus przewalskii". Among living horses, perhaps the Yukon horse most closely resembles the Przewalski's horse ("Equus caballus przewalskii") from Mongolia . The upper foot bones (metapodials) of "Equus lambei" are slender compared to Przewalski's horse. The bones of "E. lambei" also closely resemble the proximal phalanges of "E. caballus lenensis", an extinct species from the late Pleistocene of Siberia . Along with steppe bison ("Bison