Originally Posted by Mochachino
My youngster went through a nip phase and I would give him a pretty severe lip twist and I would yell. He would do it when I was leading him so I would make times and set up the situation so he would try to nip me and I was prepared to react instantly and forcefully. I would put my hand closer to his mouth and just wait and watch. A few probably 4 lip twists with increasing force did the trick. I still test him once in awhile with my hand close to his mouth while leading, and I can see the gears just a working in his brain, and I can see his eyes just thinking about it, and he chews, but he won't do it anymore. You need to be much more forceful than slapping and petting, as with that type of reinforcement he is going to become more forceful with his mouth with you and others and that can be dangerous. I would recommend someone helping you with this, but at the same time he needs to respect you.
He doesn't do it when he's tied or I'm leading him, but if I'm working with another horse or have my back turned, he'll bite at the back of my jacket & tug, then he'll start biting at other area's until I run him off.