When a donkey and a horse have a baby, it's a mule. A mule, however, can not make babies — since it has an odd number of chromosomes (63, to be exact).
While donkeys can be notoriously stubborn and horses can be fragile, mules are sturdy, patient, and live long lives. Mules always have a donkey father and a horse mother, while the opposite combination (a female mule and male horse) is called a hinny, and it's less common. A completely unrelated definition of mule is a slip-on shoe or slipper. This second meaning is from the Latin mulleus calceus, the footwear of Roman patricians, "red high-soled shoe."