There is an excellent story about an orphaned foal that was at Tufts. The reality is that horses know way more about horses than we do. We were 'given' a foal a couple of years ago and like everyone we felt that nutrition was the most important factor. It isn't, it is socialization with other horses not with people. Don't get me wrong, I can't get our horses out of my side pocket, they come when we call and they love to visit with us, but they live as a herd. Our foals go out with several adult mares the day after they are born, they understand respect, unlike humans they 'get' non verbal communication. They are also very flexible and bounce. Oh we cringe when they get bumped but that is part of their life. The worst is when they are about 2 months old and decide to run like maniacs kicking up and bucking, someone always gets kicked in the head.
The down side of not putting foals with other horses far out weighs any thing that might happend to them, the emotional well being is probably, for a social animal, the most important factor in their life.