Another thing you might want to do is to put him in a separate pasture or corral at first (if you have one), not by himself, but with one of your other horses, I think you said you had 3, who is the lowest in the pecking order. It's best if it's somewhere where they can all see each other but have a fence between them. That way he doesn't have 3 horses ganging up on him and beating him up (and they WILL) right off the bat. He can get accquainted with a less dominant horse first and kinda buddy up for a day or so, then turn them all out together. They'll still run and kick and posture at each other, but it won't be as bad and a whole lot less likely that any of them will get hurt or run through the fence.
When I first brought Bob out to be with Blade and Tennessee, I thought Blade was going to kill him. It shocked the heck out of me. Blade was just 2, but still a stallion. He was still making baby faces when Tennessee (16 yr old gelding) looked at him hard, but when I put Bob in there, it was WAR. They both ended up going over/through the fence. Luckily no one was hurt and I caught them both before they made it out to the road. I took Bob back to his mom right then and didn't bring him back until after I had Blade gelded and taken to the trainer's. That way Bob and Tennessee had a month to get to know each other and Bob had time to get used to being without his mom before Blade came back. It was kinda funny though when Blade came back because it seemed paybacks were in order. Bob ran HIM all over the pasture and tried to mount him like he was a girl. They finally settled down after about an hour. So, it can be a tricky time, just be careful.
Horses never cease to amaze me and it's so difficult to predict what they are going to do. When I brought Winnie the mare home I kept her in a round pen in the other horses pasture so they could get to know each other but not hurt each other. None of my horses are territorial (or so I thought) so after a few days I let Winnie out with the crowd. They were all fine for a few minutes and I was getting ready to leave them alone when Tuffy went berserko. I thought he was going to kill Winnie so I took my chances and went in there to get her out. I was stunned, I have never seen Tuffy behave that way. He's not even top dog in the pasture, he's second in command. When the top dog got in between Tuffy and Winnie (I swear he was trying to protect Winnie) Tuffy started kicking him for all he was worth and the top dog backed off, a very unusual happening and hasn't happened since, the pecking order is back to where it was.
Good advice to be cautious introducing new horses to a group. Caution is best, and patience. We are always excited to see how they are all going to do together and eager to push it faster than it should go.
I too, was surprised at the behavior of my Bay who I thought I knew so well. I knew he was studly and aggressive to other geldings, but thought Bay would accept and adore another mare easily, and he did not, if you recall....
Best to introduce them for a few days through a barrier they can't kick through and get caught up in. My safest spot was this gate and wood fence where my paddock and pasture meet. The only thing not perfect about this set up is that there is wire field mesh on the wood fence that I worried about, but lucked out on, there was no striking or kicking through it.
We should have left them like this for at least another day.... three days, when they are really passive and ignoring each other, would be better... When I turned them out together, the new mare still got beat up by Bay.
I would also seperated them so each of them is alone when introduced to the new horse. There a herd dynamic that happens where even the pony, protected by Bay, took part in chasing her around... this was after most of the actual rough stuff was over, this was the behavior of the next few days, establishing the new pecking order.
