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,
Male pregnancy Male pregnancy Male pregnancy is the incubation of one or more embryos or fetuses by male members of some species. Most species that reproduce by sexual reproduction are heterogamous—females producing larger gametes (ova) and males producing smaller gametes (sperm). In nearly all animal species, offspring are carried by the female until birth, but in fish of the Syngnathidae family (pipefish, seahorses and the leafy seadragon), males perform this function. Human males incubating fetuses are a subject of popular imagination and a recurring theme in speculative fiction. In some very rare genetic conditions, genotypical males develop as female and possess the