As I said, there could be a number of things at play here. You said you had trouble getting June to move in the round pen; she may be a bit slow about responding to pressure, so Phoenix may be ramping things up to drive the point home. She's young, he's not; IME, young horses frequently get rather rough treatment from their seniors. People may prefer horses with flashy paint jobs, but a lot of horses seem to hate them; bay/brown/chestnut/black horses often are the highest ranking in a herd, with lighter colors being lower down or even ostracized (I once heard about a Thoroughbred stallion that had a very strong dislike for gray horses, if he had to breed a gray mare, his handlers had to entice him with a mare of one of his preferred colors first, then switch mares at the critical moment).
If Phoenix was in a confinement situation with limited food (for example, a 1 acre field, 6 horses and 1 round bale), he could have food aggression issues. All horses push each other around to some extent; food aggression is really just an escalation of natural behavior to a level that is excessive and dangerous. Is there some resource that he seems to be trying to keep the other animals away from, or is it just his space?
Some animals are more diplomatic about their places in the pecking order, and some are just pills. I had a goat that I swear, if there were 4 walls and one door, I don't care how big the space was or many animals you might have, that space could only hold one goat - him.
I've seen horses do the same sort of thing about shelter - the run-in shelter at the barn could easily have sheltered all 10 or so horses in the pasture, but even in a downpour, I never saw more than 3 under the roof at a time (usually the 3 highest-ranking animals in the group).
On at least one of his videos, Rick Gore mentions the problem of run-ins or stalls that lower-ranking animals can get trapped in when a more dominant animal comes in behind them. I've had that problem myself. Latte and Sunny have only their barn for shelter; I leave the stall door open all the time. The stalls are 12' x 12,' you'd think that would be enough room for two animals that like each other to stand in - but it isn't.
We had an incident about a year ago when my husband woke me in the middle of the night, having heard all kinds of banging around in the barn. He didn't know what was going on, but they are my horses, he figured I ought to know about it. We got some flashlights and went out to investigate. Nothing appeared out of the ordinary, so we went back in the house.
Next morning, I saw Sunny standing near the fence, and she had no less than 12 clear marks from Latte's feet all down her right side. Some were just dirt, some were clearly puffy and bruised, and some were scrapes with both hair and hide taken off. Apparently, Latte had cornered Sunny in the stall and managed to land that many kicks before Sunny could make it out the door. And this is the buddy over which Latte has a fit any time she can't see her! Sunny is usually healing up from some contact with the Big Blonde, but that was the worst. With friends like that, eh?
