What is the offspring of a male horse and a female donkey called?
A hinny is a domestic equine hybrid that is the offspring of a male horse, a stallion, and a female donkey, a jenny. It is the reciprocal cross to the more common mule, which is the product of a male donkey, a jack, and a female horse, a mare. The hinny is distinctive from the mule both in physiology and temperament as a consequence of genomic imprinting.
Hinnies are difficult to obtain because of the differences in the number of chromosomes of the horse and the donkey. A donkey has 62 chromosomes, whereas a horse has 64. Hinnies, being hybrids of those two species, have 63 chromosomes and are in the majority of cases sterile. The uneven number of chromosomes results in an incomplete reproductive system. According to the ADMS: "The equine hybrid is easier to obtain when the lower chromosome count, the donkey, is in the male. Therefore breeding for hinnies is more hit-and-miss than breeding for mules."
The male hinny or mule can and will mate, but the emission is not fertile. Male hinnies and mules are usually castrated to help control their behavior by eliminating their interest in females.