A 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. There is much speculation as to the size variances among hybrids, some believe it is more physiological, others claim it's genetic. 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.