Sounds like Seven has been training YOU!
"I don't do that." "I will nip you if you do something I dislike."
Excuse me?!
Who's in charge here?
I know what you're going through as my Kody is the world's most bull-headed horse when he wants to be, but my Lord he'd think the sky had fallen on him if he ever tried that.
I'm all for the sort of touchy-feely training that drives Myrna nuts
but Seven is just being a total brat and you're letting him. He needs a "come to Jesus meeting" and to get a little respect for you as the higher-ranking herd member.
Take a short whip out with you the next time you do a leading lesson and carry it in your other hand projecting out behind your back towards Seven's hip. If he doesn't walk off when you do, give him a tap with the whip on his hip. Should he do something stupid like try to bite you, back him up 90 miles an hour and make him think he's going to die. You have about five seconds to convince him he just made the biggest mistake in the world, then it's over. Give him a second to think about it then return to your leading posture and calmly ask him again to work. He does not get a choice in this stuff. He is not the boss. You are.
When he does it right, we replace the metaphorical stick with the metaphorical carrot. Praise, praise, praise, and he gets to quit the annoying task for a few minutes. For a lazy horse that's the best reward! They learn it's much easier to do the task right the first time than to have to do it over and over and over....
Leia