However, in the trailer he gets aggressive with the other horses.
He went after my sister's horse in the trailer (we have a 2 horse straight load) when they were unloading. He was untied 1st to back out, and that's when he bit her mare. He also did the same thing to another friend's gelding in her slant. I guess he also kicked at another horse in the straight load during loading. He was the 1st one in.
How do you fix that? Any ideas.