My two cents, It may be good that the poster is sending this horse to a trainer, but the horse is trying to establish dominance over the owner. My point is the owner is the one that needs to work the horse. Longshot you need to have the trainer show you how to round pen this horse, know what signs the horse gives you, ie., chewing and licking. I agree that you need to be close to the horse to discpline it, but have the horse in a halter and lead rope or line. If the horse does something disrespectful you have about 3 seconds two respond to it or the connection between the horses act and your response will be lost. Much longer then a couple of seconds and the horse won't know why you are slapping it. Your horse will treat you different then it will treat others. We have 6 horses I have 4 my wife has 2. We handle each others horses (lead to turn out, feed, etc.) when I feed, my horses know to stand against a wall and not crowd me, her horses will pen thier ears get a sour look and try to steal a bite (I make them move back). When she feeds, her horses back off and wait nicely ( as nice as a horse can be when waiting for grain) but my horses give her crap every time.
When your round penning the horse use a whip or rope as an extension of you hand, this way you can put pressure on the horse (to establish dominance) at a safe distance. All horses that turn their butt to you are dangerous all the time. in a stall, round pen or pasture.
I was leading a mare and a day old to a turn out area last spring, let mom go, she did what she was trained to do but the day old started ripping around. I stood there smiling watching my new foal as it ran up to me turned and missed my face with a double kick by about six inches. I learned it is never to early to learn some manners.