described. The validity of some of these species is questionable and a matter of debate. For example, Equus niobrarensis is likely synonymous with Equus scotti, while Equus alaskae is most likely the same species as Equus lambei, which itself may be a North American form of the living Equus przewalskii. DNA studies on American horse remains found frozen into permafrost have shown that several of the supposed American species, and the European Equus ferus, are actually one highly-variable widespread species. , as if the evolutionary process of speciation was persistently being frustrated by large herds of the horses moving long distances and mixing, carrying their genes about with them. Equus alaskae - Alaskan horse Equus algericus Equus capensis - Giant zebra Equus conversidens - Mexican horse Equus dalianensis Equus fraternus Equus francisi Equus giganteus - Giant horse Equus lambei - Yukon horse Equus lenensis - Siberian horse Equus latipes Equus mauritanicus - Saharan zebra Equus namadicus Equus neogeus - often placed in a separate genus, Amerhippus Equus niobrarensis - Niobrara horse Equus occidentalis - Western horse Equus ovodovi - Ovodov's horse Equus scotti - Scott's horse Equus semiplicatus Equus simplicidens - Hagerman horse Equus sivalensis - Indian horse Equus stenonis - Stenon zebra Equus yunnanensis - Yunnan horse Domestic species Equu caballus - Domestic Horse Equu asinus - Domestic Donkey Hybrids Equine species can crossbreed with each other. The most common hybrid is the mule, a cross between a male donkey and a female horse. With rare exceptions, these hybrids are sterile and cannot reproduce. A related hybrid, a hinny, is a cross between a male horse and a female donkey. Other hybrids include the zorse, a cross between a zebra and a horse and a zonkey or zedonk, a hybrid of a zebra and a donkey. In areas where Grévy's zebras are sympatric with plains zebras, fertile hybrids do occur. Ancient DNA identifies the Bronze Age kunga as a cross between the Syrian wild ass and the donkey. Biology Physical characteristics Equines have significant differences in size, though all are characterized by long heads and necks. Their slender legs support their weight on one digit (which evolved from the middle digits). Grévy's zebra is the largest wild species, standing up to and weighing up to . Domesticated horses have a wider range of sizes. Heavy or draft horses are usually at least high and can be as tall as