Horses within a herd usually buddy up. It's common to see pairs of "preferred associates" mutually grooming, defending one another when the need arises, and standing head-to-tail swatting flies in the summer.
Choosing a buddy for your horse is almost impossible because it's difficult to predict whether two equine personalities will get along. Sometimes best pals are very much alike. Sometimes they're complete opposites. For instance, alpha mares often select lower end of the hierarchy, and it's not unusual to see a rowdy youngster prefer the company of a more placid, older herdmate.
There are ways to manage a herd to encourage equine friendships. It helps to have an even number of horses in a field. Avoid a group of three, if you can. Two of them will nearly always pair up, and the odd man out will continually attempt to put himself into the mix. In addition, use a field that has plenty of lounging space. Room to roam encourages stable partnerships.
Before introducing a newcomer to your herd, put him in a small enclosure with another horse, preferably one without a best buddy, for a few days. After they've bonded, turn them out at the same time. At best, the newcomer will be protected from the herd by his good pal. At the very least, he'll recognize one herdmate and take a bit less abuse during his first few days. Some horses are loners, content on their own. They don't seek companionship, have never been herdbound or whinnied anxiously when moved to new surroundings. There is no need to worry about a loner unless his aloofness is denying a needy herdmate of friendship.