I keep my two geldings at home. The 11 yr old is dominant over the 22 yr old. Both are warm, friendly horses with humans. Popping one of them into a stall is sometimes the most expedient way to being able to do whatever it is I’m doing. Sometimes though, I’d like to be able to change a blanket or fly mask or check a hoof without having to physically separate the horses. Of course all is well if I’m messing with the younger horse as the older horse will keep his distance (unless I’m riding then he can be a pest lol), but if I’m messing with the older horse I had the same problem where the younger horse thought he should be able to push the older horse out of the way and monopolize my attention.
First things first. This is a safety issue. Can’t have horses bowling each other over while the humans are in the mix. So physical separation either by tying or stalling is the safest option. Secondly, we have to teach a horse a skill before we can utilize a skill. So we need to be able to reliably influence the young horse’s behavior in a training environment without having a halter on him before we can reasonably expect to have an influence on him in the real life setting of working with the older horse in the loose group setting. Sure any yahoo can influence any horse by getting big and loud. And we’ll absolutely do that if we need to preserve our safety.
My younger horse (that’s much older than your younger horse) has extensive ground work skills. Part of that ground work is fine tuning the much maligned “turn in” where the horse turn in to face and approaches the handler after a “lunging” exercise. The fine tuning I’m speaking of is controlling how far the horse turns in / how close he approaches. Like any training program, we may have to get to a “loud” cue at first to influence the behavior but through consistent progressive training we teach the horse to respond to a much lighter / more subtle cue. A very similar training occurs teaching the horse to back up / away from our personal space if the horse has creeped up on us. At any rate, all that is done on a line and at liberty without another horse in the mix. Once the horse is responding reliably to my change of energy, my intention, “the force of my regard”, I can then utilize those same skills to prevent my young horse from bothering my old horse while I put his fly mask on or whatever.
So the short version is, I give my bossy young horse “the look” and he keeps his butt over there. It works because he’s been trained to respond in that manner to that look.