Cross bred between the male donkey and female horse, the mule is equipped with great athleticism and intelligence. Although in some situations you may come across a "hinny"(the result of breeding a female donkey and a male horse). For best results of a hard-working, sedulous animal, the hinny is not a suggested breed.
Mules have only 63 chromosomes, thus making them sterile. Though this is true, a mule should be appreciated for their hardiness and docility. Mules skin is less sensitive and more resistant than of a horse. This allows them to endure harsh climate conditions like the heat of the sun or heavy rainfall. Apart from climate, this genetic trait allows mules to comply with heavy equipment (unless the equipment is poorly fit). Their hooves are harder than horses and they are able to pull a heavier load than of a horse of the same size. Mules are the ideal hybrid animal for moving through trails of any height and size. They were, and in some places still are, a very valuable piece of farming.
Mules understand a lousy job when they see one. Through patience, kindness, and understanding, "starting" a mule requires building trust and partnership. It is only an abundance of common sense and a strong desire for self-preservation that make a mule more inclined to resist.
Maybe this is why the phrase "stubborn as a mule" has stuck with them for so long…