What is a mule? A mule is a cross between a mare and a jack (donkey stallion). A cross between a stallion and a jennet (female donkey) is a Hinny. Mules are male and female, just like horses, with all of the sexual organs, so females (called mollys or mare-mules or jenny-mules) do cycle and males (called johns, jack-mules or horse-mules) need to be castrated or they will get stud-y. However, donkeys have 62 chromosomes and horses have 64, so mules have an uneven number, 63, which makes them a sterile hybrid. They do not reproduce..... lmao. damn Mule.. you poor bugger