I told my friend that I get lots of good advice on here, so she asked me to post about this and see if it helps her. I'm not sure if there is a "proper" name for this, but I just call it herd-bound. The horse is a middle-age gelding that was a rescue from a lot where the horse was kept alone for I don't know how long. Years, probably. I personally don't know if that plays into how he acts or what.
Every time he's alone, he freaks out. He had to stay in his stall to wait for a vet to come give him a check up, and she ended up letting him go into the pasture for fear he'd hurt himself. The stalls are right next to the pasture and the other horses (two mares) could easily come up and stay beside him if they wanted. He could see them, too, but he pranced and tried to rear and circled and rushed the stall door to the point we thought he was going to try jumping it. For his safety we let him out and just caught him again when the vet came. The three horses (him and two mares) are all that are out there, as this isn't a boarding facility but just her own private home.
We tried keeping one mare in the stall beside him and letting out the other, but he still freaked. We put the two mares in stalls far away from him and he didn't seem to care, because they we all "in".
Taking him alone is impossible, even if its within sight of the others. When he's with them, there are no problems and they all get along. But in situations where he has to stay in his stall, like for sickness or injury, its becoming a huge issue.
Is there a way to modify his behavior?