Well.... I always had this problem with my qh. Whenever I ride side-by-side with someone on trot or lope she finds a moment to kick the other horse. I can NOT catch the moment as she gives no warning (she's extremely smart): pinning ears and good kick on side on speed go in same time. She's certainly just shows her dominance, and she does it to ANY horse (even the one she seems to really like in any other situation) if she can catch the moment. She also knows when I watch out her so she always picks time when I'm concentrated on something else. Anyway... Any ideas to deal with it except using the red ribbon on tail?
The problem is people (especially with dominant horses) usually don't believe she can do it, so they are pretty careless about the situation. And YES, I know it's my (my horse's) problem, not those people, but it's hard to manage when they try to speed up or go side-by-side.