This horse is my horse and the concern is coming from other boarders at the barn, now I know what I would do if I was there and he did this to me but he never has ever done this before. I have owned him for 4 years and there has been two incidents in the last 6 months of him trying to go after horses that are being led through a gate. Today he charged at another owner who was leading her horse through the pasture gate on her way with some others for a trail ride. I suggested taking a lunge whip out with her and chasing him away with it , getting bigger than he is to put him in his place. He is Alpha in the herd and I think it bothers him to see his mates leaving the pasture without him. I have not seen him do this as I was not there when it happened but I think that everyone(boarders) needs to be on the same page so that it doesn't turn into something more dangerous.
Please only helpful suggestions, it really concerns me that he is doing this and only doing it at certain times.