^ This is how the babies at my barn (well not my
barn, but where I board) are raised. Early in life they spend all their time in the stall with their moms, with daily turnout with their moms.
When it comes time for them to be weaned, they always are kept with another baby while they grow up and learn to be horses. They learn how to be good in stalls... are usually given a prime spot where they get fed treats and are handled all the time by all kinds of people throughout the day, every day. They are turned out together and so forth.
And then over time, they are turned out with other horses and my barn lady and all of the people who work at the barn keep an eye out for any personalities that do not get along together.
So generally by the time they are all grown up, they have their own pack that they are turned out with on a daily basis and/or longer. And they prefer being with other horses out there, because that was how they were raised.
Definitely, you can raise a more solitary minded horse by not doing as much nurturing and socialization, and handling.... but it's not always the best thing.
My personal THING or what I love the most about my guy and all the other horses at this barn, and it is a huge credit to my barn lady - these horses are closer to being dogs than most. They genuinely like and trust all the people who come into the barn and enjoy being handled. And while there are personality clashes (my horse can never be turned out with one of his brothers, because they will try to kill each other), they generally are a peaceful pack because of how they were raised.
Should add, another attractive thing that keeps me boarding at this barn - every horse is very socialized about cats and dogs. There may be a unpredictable element still as far as whether a young horse (or a mare) will get skittish about a dog being underfoot. But generally, when my dogs are out in the gelding's field with my guy and the others - I'm not that concerned. Because again, these are very socialized, handled, and mentally sound horses who have been inside the barn with people and dogs and have that trust.