I saw this kind of thing happen with a 14-year-old Paint gelding called Dakota. I'd known him for several years, and he'd always been near the bottom of the pecking order in any group he was in, and apparently content to be there. At this time, he was in a pasture with my two QH mares (Latte and Sunny) and a Quarab gelding called Mojave. Mo and the girls were a group, and Kota was a sort of satellite group of 1, and they all seemed to be satisfied with that arrangement. Then the barn owner moved 4 Appaloosa mares into the adjacent pasture. Kota became absolutely besotted with those spotted females. He paced the fence, calling to them, and violently attacked the other three if they got near them. I once saw Kota get in a kick fight with Latte, the horse that we all knew had the biggest attitude on the place, and win. We were all stunned to see mild-mannered Dakota acting like this. I even had to set up an extra water trough, because the one that was in place was on the fence line, and he wouldn't let Mo or the girls near the fence.
Things entered a new level of craziness when a new horse, a 17-hand Warmblood gelding named Unity, came on the place. Unity was turned out with the Appy mares, a situation that pleased him immensely, but drove poor Dakota to distraction. Kota and Unity bickered over the fence for days, but by the grace of God I was there when they knocked the gate off its hinges and Kota got in the pasture with Unity and "his" girls. One of the scariest things I have ever done was to get between those two squaring-off band stallion wannabe's and lead Kota off before serious damage got done.
That situation only got resolved when Dakota's owner moved him to another place (financial reasons). I hated to see them go, but in a way, it was a relief; the fence dividing those two pastures was in pretty sorry shape, and it was only going to be matter of time before it got knocked down when nobody was there to intervene.
As uncomfortable as it is for me to see Joe looking like that, it must be ghastly for you. Is there any way (other than rehoming) to break this up and give him (and you) some relief?
Pearl has figured out that she is also in feeding order. At first she was all over the place, frantic for Feed. Sparkles is first, then Prince, Joe and then Pearl. She has quit blasting through the barn and goes around to her end. LOL
Got all their hooves trimmed a couple weeks ago. Winter hair is coming out, she should slick out nicely.