I'd say that it totally depends on the animal. In my experience the older horses that have "been there and done that" are more at ease with being alone. I bought a 12 yr old QH that had been alone most of her life and when put in a boarding stable she was best off put in a pen by herself. When turned out with others it took a long time for her to adjust and she was hurt several times ( not only by others but from her own silly actions). I eventually brought her to my home and had her by herself and she fine with that. When I brought home a mini gelding ( age 15), that had been in group settings all his life he and her pretty much left each other alone.
I then sold the QH and the older gelding was OK with being alone. Now however, he's adapted to being with other minis and if they leave for a show without him.....well, let's just say you can clip him and wash and do just about anything to him if he thinks he's going to "get to go with the others".
The worst horse I've seen is a young yearling who goes crazy if you move the horse in the next stall for grooming, let alone leave him at home when leaving for a show with others.
I then sold the QH and the older gelding was OK with being alone. Now however, he's adapted to being with other minis and if they leave for a show without him.....well, let's just say you can clip him and wash and do just about anything to him if he thinks he's going to "get to go with the others".
The worst horse I've seen is a young yearling who goes crazy if you move the horse in the next stall for grooming, let alone leave him at home when leaving for a show with others.