Mules come from crossing a female horse with a (male) jackass. A horse's ass would be an ass kept for breeding purposes. Considered a low form of life, like a gigolo.
Someone who's got most of their true muscle between their ears, but still makes neverending desperate attempts to physically dominate people with body language (especially in crowded public settings), often resulting in police showing up to threaten people who don't think life is like a poker tournament or a trip to Vegas. Also known as an instigator.
He/she was a horse's ass, but not everybody was ignoring him/her like she thought, many of them were rejecting him/her, but he/she could never see that, all the muscle between the ears was dense.