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 (genus) each other both visually and vocally. Human activities have threatened wild equine populations. The word "equus" is Latin for "horse", and is cognate with the Greek ἵππος ("hippos"), "horse", and Mycenaean Greek "i-qo" /ikkʷos/, the earliest attested variant of the Greek word, written in Linear B syllabic script.; compare the alternative development of the Proto-Greek labiovelar in Ionic ἴκκος ("ikkos"). The genus "Equus" was first described by Carl Linnaeus in 1758. It is the only recognized extant genus in the family Equidae. The first equids were small, dog-sized mammals (e.g. "Eohippus") adapted for browsing on shrubs during the Eocene, around