I have a 5yrold mare who is also agressive to others when I am visiting. She's not as bad as she used to be now because she has learned that I don't like it. Now, if she does it, I just have to frown at her and it's over.
What I did: everytime the behavior came on, I would be angry with her. If I had a halter and lead on, I would scold her and turn her in the opposite direction. If I was just visiting and had no tack to work with, I would make some kind of agressive move and/or sound -- if I was in proper position, I would push her away. Anything really, to make her understand I wasn't happy with her behavior. If I didn't feel I had control that I could contain the situation, I would be vocally agressive and chase her away. When she was far enough away, I would go and visit the other horse. If she came back, I'd chase her away again. Then I would leave without another look at her.
Like I said, she's much better now. I think you are on track for nipping this jealous / territorial behavior at a young age. Especially in a colt.
Last edited by NorthernMama; 12-28-2008 at 11:00 AM.
Reason: improve phrasing