The first time I put two stallions together I was very green at this and my friend assured me so long as the field is big enough, so long as you stay calm- ya da ya da etc!!
I do not panic easily, I have to say but, having done this I was at least prepared, with a big stick, to part them if needs be (they had been introducing themselves for two weeks on either side of the fence line, and had settled down)
I was NOT prepared for the ferocity of the attacks and, just like Lisa, I was surprised at the relative lack of noise- it was even more frightening than if they had screamed and yelled.
First lesson- screaming and yelling means they probably AREN'T going to try to kill each other!!
I got them parted and apologised profusely to the older one, who had got beaten up but not badly.
Funnily enough, I do still run entires together but I have learned to watch the body language- you know straight away if it will work.
I had two, 2 year old colts together last year and they were great, no problems at all.
I put a weaned colt in with the older horse and everything was great, no problems- I did actually try the whole lot together- with an electric wire down the middle, though, and it was immediately obvious it was not going to work- the older stallion became quite rattled, and the young colts were being VERY rude to him- lack of respect is often at the bottom of all the fights.
The older horse, in this posts case, should NOT have attacked a yearling colt, I think I would think long and hard about the temperament of a horse that reacted like that, the colt is not a stallion, he is a baby- but he had obviously been very rude to the older horse for some time- some stallions, especially when they are related to the colt, will tolerate this, some won't.
If you do not know what you are doing it can be a train wreck, though.
Carlos, my young stallion (colt really but he is "working" at three so I will call him a stallion) will tolerate ANYTHING.
He is going to love running this winter with Toby who will be a weanling, he will allow him, as he does at the moment, to steal his feed and bite his backside.
Carlos is just a very laid back, tolerant, stallion.
Rabbit, my dear little, gentle as a mouse, Rabbit, would chase Toby beyond any sort of sense- the strange thing is he would not actually attack him unless he was on the other side of a fence, just run him down until he keeled over (Rabbit would keel over first but I do not think that would stop him)
I do know of people who run stallions on the other side of a fence, all year round, but I am guessing the stallions have come to an arrangement and the fences are strong!!