There's a joke in here somewhere about a horse's ass but I'll be damned if I can come up with it.
From Yahoo :
Mules are bred by mating a male donkey with a female horse. The breeding success is about the same as among horses alone. Mating a male horse with a female donkey produces an animal called a hinnie. Both mules and hinnies can be either male or female, but they are almost invariably sterile.
It's not going to be long before this cloning thing gets out of hand.