Zamric said:
The mule is an intrageneric hybrid between a male donkey (Equus africanus asinus) and a female horse (Equus ferus caballus). The rarer hinny is obtained by crossing a male horse with a female donkey. In both mules and hinnies, males are always sterile. Rarely, however, female mules and hinnies can be fertile, probably because female mammals have two X chromosomes, while males have only one (Haldane's Rule).
People have been producing mules (and occasionally hinnies) for centuries, because although sterile, they have the surefootedness of the donkey, but the tractability of the horse. Also, except for their sterility, they are very healthy (hybrid vigor).
Intrafamilial hybrids, however, are not healthy (at least in mammals). A hybrid between an Asian elephant (Elephas maximus) and an African elephant (Loxodonta africana or L. cyclotis) dies during infancy at the latest.
