Mule is the cross of

Mule is the cross of Mare × Ass.
The mule is a domestic equine hybrid between a donkey and a horse. It is the offspring of a male Ass (a jack) and a female horse (a mare).
A female mule that has oestrus cycles, and so could, in theory, carry a foetus, is called a "molly" or "Molly mule", although the term is sometimes used to refer to female mules in general. A male mule is properly called a "horse mule".