Source: George Washington's Royal Gift
Mules are the offspring of a male horse (stallion) and a female donkey (jenny). A donkey is also known as a jackass, and a male donkey is a "jack." The offspring of a jack and a female horse (mare) is known as a "hinnie."
Donkeys were domesticated from wild asses in tropical Africa, and later imported into Europe from North Africa. Today's donkeys are genetically related to just a few male asses, so the people who domesticated the species about 6,000 years ago were careful in their breeding program.
Horses, zebras, and donkeys share a common ancestor. All three are in the genus Equus, which emerged 55 million years ago - about 10 million years after the dinosaurs died. The modern equines developed about 4-4.5 million years ago, and:1
Because horses and donkeys/asses are separate species today, their mule offspring are sterile. Maintaining a population of mules requires continually breeding horses and jackasses. Male mules are typically castrated to reduce hormones and make them easier to manage. Castration does not reduce the value of a mule; it has no chance to produce offspring anyway.
Mules were popular farm animals prior to development of the tractor at the start of the 20th Century. Mules are as strong as a horse when pulling machinery or carts, but require less food. in addition, mules often live longer than horses, and are less likely to suffer from a disease. They are intelligent animals, and their reputed stubbornness is often a recognition that the requested behavior in unwise because it involves too much risk of harm.
The American Mammoth Jackstock breed evolved from imports of different breeds of jackasses from Europe, particularly the Catalonian jackass from Spain.2
George Washington was one of many plantation owners who imported jacks and jennies, but importing the highly-prized Spanish mules required special permission from King Charles III. Washington sought permission during the Revolutionary War, but his requests were not successful.
International politics gave him another opportunity in 1784. That year, Spain closed the Mississippi River to American traffic. Settlers west of the Appalachians depended upon sales of their farm products at New Orleans, and the river closure was expected to depress significantly the value of land in the Ohio River and Mississippi River watersheds. The Confederation Congress voted unanimously in August 1785 to instruct John Jay that any treaty he negotiated with the Spanish must include reopening the Mississippi River.
Spain closed the river as part of an effort to "contain" American expansion, similar to US foreign policy to contain the expansion of the Soviet Union between 1948-1990. Spain feared growth of American settlement would threaten its hold on East and West Florida, plus territory is claimed both east and west of the Mississippi River. The Spanish envoy to the United States, Don Diego de Gardoqui, recognized the initial united response by the 13 states could be fractured by adroit diplomacy, including gifts and even bribes to key Americans.
Gardoqui's primary strategy was to offer a trade treaty that would benefit New England and Mid-Atlantic states, allowing exports of timber and fish that would enrich American merchants and ship owners. Spain sought to fragment political blocs in the Congress, resulting in a change of instructions to Jay. If the end result was dissolution of the United States, that too would meet Spain's foreign policy goals.
As part of a charm offensive, Gardoqui arranged for King Charles III to send a stallion to John Jay and two mules to George Washington as gifts. One of the two mules died while being shipped across the Atlantic Ocean. The other arrived in Boston, but Washington had it shipped to Mount Vernon.
What Washington named "Royal Gift" was a valuable jack, but he was reluctant to mate with mares. The ultimate solution was to trick Royal Gift with a bait-and-switch process. He was exposed to female donkeys, and when excited at their presence the jennies were removed and a mare brought forward for the mating.3