I'm going to have to see some solid proof of this before I buy it. As it's been pointed out, the current record tallest horse is 20. And if you google pictures of him he is an absolute MONSTER. If he is indeed 21 you'd better be calling Guinness. And post pictures for those of us who are skeptical.
In my experience, liking often leads to nipping and biting. So, I'd either discourage it or keep an eye on it so it doesn't escalate.