Mules are used throughout the world as beasts of burden, including parts of Asia, Africa and Central and South America. A mule is the offspring of a male donkey and a female horse and the variations amongst mules is a direct result of their individual lineage.
Below is a poster featuring Moroccan mules. In 2003, a mule in Morocco made headlines throughout the world as a female mule gave birth to a colt. The reason this made headlines is that mules, as a whole, are infertile. There are only as few as 60 documented incidents of a female mule giving birth. There are no documented incidents of a male mule being able to sire any offspring, so it is believed that all male mules are infertile.