Mules are smarter than horses and can haul heavier loads but you have to treat them right. Mules "know" how much they can haul and will refuse to haul more than that. All the whipping in the world will not get a mule to haul a load too heavy. Hence, stubborn as a mule.

Mules also do not need as much water or feed as horses so they were preferred as draft animals. We still have not found just why the word "skinner" was used. No one refers to "horse skinners" or "ox skinners". I don't believe that it was the whip thing---whip a mule and you have Trouble with a capital T.