When donkeys and horses mate, their offspring are hybrids. The most common horse-donkey hybrid is the mule, an animal whose mother is a horse and whose father is a donkey. When the hybrid's father is a horse and its mother is a donkey, the animal is instead called a hinny.
Horses and donkeys are both classified as equines, or animals with stocky bodies, long necks and single-toed feet called hooves. Horses and donkeys are closely related enough to mate, but because they have different numbers of chromosomes, their hybrid offspring are almost always infertile.
Mules and hinnies are often grouped together due to their similar parentage, but they do differ somewhat based upon which species was the mother and father. Hinnies are generally smaller than mules, in part because they were carried in the womb of the smaller species. They also have a more donkey-like gait and more donkey-like hooves. Otherwise, the two hybrids are almost impossible to tell apart, and many people call hinnies "mules" to simplify things.