Whenever, I add horses together I watch them for a few minutes. Do during daylight hours because that way you can check on them every few hours. Expect some squealing, threats and maybe some chasing of one another. If its multiple horses make sure they don't get the new horse into a corner and kick the ever loving daylight out of it. Most horses will sort themselves out in 20 minutes or so. What you need to watch for is that no one horse is dominating the resources, Ie. Keeping another away from grain or more importantly water. I have found that some horses calm down when I feed them shortly after introducing a new horse, they sort of get distracted by the hay. The problem some can work up a good sweat while running each other around and it can make you worry about feeding them after. If you think their is a problem, don't rush in and risk getting hurt. Think and strategize, you don't want an injured horse and you getting kicked in the head.