I agree with the hind quarter yielding-if he's paying attention to you
asking him to yield, he'll have his mind less on the other horse. I had
great success with this technique with both of my mares, who wanted to kick
other horses behind them. I always teach it during my ground work, it's one of
the "7 Games of Natural Horsemanship" in the Parelli program, though I think
others use this technique now too. The horse needs to see you as his leader,
then he'll listen better and worry less about the other
horses.
