Actually, I'm not sure that he is just herd sour. I think that he is every other horse, ever, sour.
This is my situation: I keep my horse at my house. I ride in both the pasture and the fields beside the pasture. I have no trailer, and there is no arena nearby. My gelding lives with my cousin's mare. I rode the mare up until I got my gelding, because my cousin is afraid to ride her mare.
I put the mare in a stall before I ride my horse- I left her out once, and it was a disaster. She ran up and down the fenceline and all he would do is follow her.
Yesterday, we had our worst ride ever- he would randomly turn and gallop at full speed back to the mare. He would not stop, at all. He drug me through trees branches and I was sure he was going to hurt himself. I was shaking by the time he finally stopped. He enjoys playing with his bit, so he might have gotten it under his tongue, but I'm not sure.
I would say he is just "buddy sour" but it's not even just the mare that he wants to be near. At the back of the field I ride in, there is a pasture that borders the field's perimeter. In this pasture are two other geldings. When we get near them, my horse will start neighing frantically and trying to get closer to them. It is so hard to drag him away.
When I rode him at his old owner's house, there were three other gelding to which he was really close. However, during that ride, he never tried to run back to them. So I am a little confused about the new behavior. I haven't owned a new horse in 10 years- ridden one, yes but this is my first legitimate new horse ownership in a while. He is four years old, almost five. I've only had him for about a month, so I know that we need more work, and he still needs to get used to his new home, but I could really use some advice about this problem.