Mules are great animals, larger and more personable than a donkey, smarter, hardier and less easy to scare than a horse, they are also healthier and have a longer working life than either horses or donkeys because of hybrid vigor. However to create a mule is no simple task, most Jacks (intact male donkeys) have no interest in mares, and most mares will not present for a Jack like they would for a stallion, and are liable to try to kill a Jack that tries to mount her unwillingly. The only way to make it work is to raise the mares and the Jacks together, so that mares see and smell Jacks as just different sorts of horses and vice versa, it's a very deliberate process, the production of mules has to be planned years in advance, though in the modern day it can be and mostly is circumvented by artificial insemination.

Keeping in mind that this is a deliberate process, not something that will just accidentally happen in a stable or in the wild, when did people first start breeding mules and what originally inspired them to do so? The only information that I can find is that the mule was already well established as a working animal and an infertile hybrid by the time of the Roman Empire, as evidenced by one of their common expressions "when the mule foals" which means the same thing as "when pigs fly."

Thank you for your answers!