HinnyA hinny is the offspring of a male horse and a female donkey (jennet or jenny). They are rarer than mules, which are the children of a male donkey (jackass or jack) and a female horse. Like the mule, the hinny is almost always sterile. The head of a hinny looks more similar to that of a horse than does the head of a mule. Hinnies are on average slightly smaller than mules. Otherwise, there is little difference between them.
Hinnies are difficult to obtain because of the differences in the number of chromosomes of the horse and the donkey. Sterility in the mule comes from the same reason. A mare mule, called a "molly," has estrus cycles and can carry a fetus, as has occasionally happened naturally but also through embryo transfer. The difficulty is in getting the molly pregnant in the first place. Nature prefers the chromosome match-up to occur when the jack is the sire and the mare is the dam. It has been known for people to let a stallion run with a jenny for as long as six years before getting her pregnant.