So I doubt we'd be able to squeeze him in too tight with another horse in a trailer. However, it could just be the circumstances he was in (he was a trail/therapy horse at a retreat with lots of beginners riding him) and he got stressed. Maybe with just one or two buddies and his own family he won't be stressed and cranky towards other horses. Only time will tell. But I'm figuring it'll be like my kids in the car on a long ride.
Actually, that might not be the case. I have a 2 horse stock trailer with no dividers. I have 3 horses that got along perfectly and one horse who was the "odd man out." He never got to be in their tea parties and always grazed separately. When they were in the sacrifice pen together, the others always chased him away.
So, I thought as you did--the others would never share a trailer easily with him. One day, on a trailered trail ride, the person who hauled my "outcast" third horse for me in his partitioned trailer, had to leave for an emergency. There I was, out on the trail, with 3 horses and a 2 horse trailer. I thought, "Nothing ventured, nothing gained. We'll give it a try."
I loaded the first two on a slant, left the door open, and the 3rd horse, the one the others wouldn't accept, just jumped right in, pushed the other two over, and they hauled like a dream. After that, we hauled him with the others from time to time, and never had a problem. My other 3 weren't nice to him in the pen and pasture, but they accepted him just fine in the trailer.