Hybrids
Horses can crossbreed with other equine species to produce hybrids. These hybrid types are not breeds, but they resemble breeds in that crosses between certain horse breeds and other equine species produce characteristic offspring. The most common hybrid is the mule, a cross between a "jack" (male donkey) and a mare. A related hybrid, the hinny, is a cross between a stallion and a jenny (female donkey). Most other hybrids involve the zebra (see Zebroid). With rare exceptions, most equine hybrids are sterile and cannot reproduce. A notable exception is hybrid crosses between horses and Equus ferus przewalskii, commonly known as Przewalski's horse.
Read more about this topic: Horse Breed