Mules are the product of mating a horse with a donkey and are born sterile, so a mule cannot be bred with any type of animal. Breeders who want mules can breed a male donkey and female horse together, while breeders who want hinnies can breed a female donkey with a male horse.
Johns and Mollies are the terms used to describe male and female mules. A male mule should undergo the castration process at the appropriate age to avoid hormonal and behavioral problems. Although mules are born sterile, the males will still have the hormones used for reproduction but be unable to efficiently release those hormones, which can lead to health or behavioral problems in the future.