Join Date: Jun 2009
Location: Alberta, Canada
He needs training. This is not an equipment issue.
There isn't a piece of tack in the world that will fix this problem.
I noticed one thing you mentioned. He used to be a rental trail horse. THAT is probably a huge reason why he is herd bound.
My girlfriends pony was a rental trail horse before she rescued him. He was the same way. Hard mouthed, herd bound. Although he was to the point of dangerous (which is why the stable sold him). He would buck/rear/roll/bolt whenever you tried to hold him back.
Horses are herd animals, when you ask them to stay behind the pack, it's makes them anxious, because naturally they aren't comfortable being alone. It makes them very vulnerable.
Your horse needs to learn to trust you, so that when he's with a group, he doesn't feel anxious when the other horses leave him. Arena and ground work and lots of alone time, just you and him, is what you need.
And of course a trainer.