Mule (Latin mulus) is a term sometimes applied to the offspring of any two creatures of different species, and synonymous with hybrid. In its most common meaning it describes the offspring of a male donkey and a female horse. The offspring of a stallion and a female donkey is known as a hinny. The mule is easier to breed and is usually larger in size and so the attention of breeders is entirely directed toward its production. Male mules and hinnies are both sterile and although it has been known for female mules and hinnies to have foals, most are infertile.
In its short thick head, long ears, thin limbs, small narrow hooves, short mane, absence of chestnuts (horny growths) inside the hocks, and tail destitute of hair at the root the mule is asinine in form. In height and body, shape of neck and croup, uniformity of coat, and in teeth it is equine. It has the voice neither of the ass nor of the horse, but emits a feeble hoarse noise. The most common colour of the mule is a brown or bay-brown - bay, or bright bay, or piebald being rare; a chestnut tint is sometimes noticed. It possesses the sobriety, patience, endurance and sure-footedness of the ass, and the vigour, strength and courage of the horse. As a beast of burden it is preferable to the horse, being less impatient under the pressure of heavy weights, while the skin, being harder and less sensitive, renders it more capable of resisting sun and rain.
The mule has been in use from early times; the inhabitants of Mysia and Paphlagonia are said to have been the first breeders. The ancient Greekss and especially Romanss valued mules for transport, employing them to draw carriages and carry loads. In the early 20th century mules were used largely for military transport.
Mules have become far less common since the rise of the automobile, motorized tractor, and other internal combustion-powered vehicles. They are still used in less-developed countries, and in certain specialized roles in industrialized nations: they can handle narrow, steep trails--such as the route down into the Grand Canyon--that are not suitable for motor vehicles or horses.
based on an article from 1911 Encyclopedia Britannica
The Mule is also a character from Isaac Asimov's Foundation Series. He is a rogue member of the planet Gaia who has the ability to manipulate human emotions. He uses the ability to take over the Foundation. He is a central character in both Foundation and Empire and Second Foundation.
A Mule is someone who knowlingly smuggles something onto an airplane, but does not know what the contents of what he or she is smuggling. E.G. a terrorist tells someone that the package contains diamonds and tells him or her to smuggle it. Yet the package actually contains a bomb.