Nicely summed up by the above posters so I will take it one step further to say there are two types of "lead" horses.
1. The Alpha dominant horse that establishes its "Boss Hoss" leadership by biting, ear pinning, and kicking.
Depending just how hateful that Alpha horse is, those methods can range in degree of agression from lip-nipping, to a couple "gentle" bite marks on someone's neck or butt, to downright laying another horse wide open with a well-placed bite or kick.
2. There is also such a thing as a "passive leader". That "alpha- type
horse will generally go along his own peaceful way, unless someone else gets really stupid, or if the other horses are confused about something and need guidance.
I know this to be fact because:
1. I have read Mark Rashid's books and learned about the passive leader from him.
2. Mark Rashid's books is when I discovered that my little Arab is my Passive Leader.
The TWH that I've owned for 17 years has always been the alpha-dominant horse and now that he's coming 21, he doesn't get near as nasty about it as he used to
I have seen my "Passive Leader" in action many times. Either when a bad storm is coming that always make my alpha horse goes nuts, so the others run to the passive horse for comfort and guidance, or when another horse is sick or injured and I keep it in a small pasture with my passive horse to babysit.
When my young horse nearly ripped his hoof off and punctured a secondary artery, I watched my Passive horse take his head back from the water tub without drinking, so the injured horse could have water first.
Once that injured horse was well and back to playing, the Passive horse said "now that you're better, all bets are off, I get my water first".
My Alpha horse would have never done that. He would've stood by the water tub all day to keep the injured horse from drinking if he'd been able to.
Herd dynamics is a wonderful and amazing thing to watch. Take a chair to the fence someday when you have time and just watch and observe
Oh and there is one more type of "alpha/passive leader horse". That would be the human and that is how we intelligent, logical Beings manage to control our horses. How we choose to use our dominant role over them is a matter of choice.
I prefer to be their Passive Leader. My crew are very good at understanding all my hand signals and pointing/grunting, and "lip nipping". Yessss, I do that too
When the vet is done giving shots and exams, turns around while getting in her truck and says "you should know that you have FOUR of the most well-mannered and polite horses I have ever worked on at one time." I know I am doing things pretty much the right way