The first thing I have observed is that adult horses are far more patient with the playful exuberance of a foal than people are. I think this mostly stems from fear, we as frail humans are afraid of being hurt, so we react in a way that must seem excessively violent to a young horse. Now that Z is big enough to spend some time with my entire herd I have seen all of my horses put up with him nipping and even kicking at them without much reaction at all, which of course I cannot allow him to do to me, but they always follow the same pattern when they have finally had enough. First comes the dirty look; you know "The Look" standard ears back, hard eyes, wrinkled nostrils, that clearly says "If you don't quit that I am going to take a chunk out of you!" If Z is dumb enough to ignore that, then comes the head shaking and tail swishing. The tail swishing is what really has impressed me, because all of the most dominant horses, Apache, Crystal(when she was with us) and Indi, use their tail as the weapon of choice in reprimanding Zeeba when he gets too annoying. I have seen all of them deliver a stinging slap with their tails rather than lifting a hoof or snapping their teeth. It is only in an extreme circumstance that the dominant horses will use strong force by biting or kicking at (they rarely connect) Zeeba. Conversely, my less self confident horses, seem to actually view Z as a threat and will get physical with him at very slight provocations. Oddly, Z shows them far less respect than the horses that use minimal aggression. This all fits in with how I've always viewed interchanges between adult horses as well, the only difference is that they seem to show more patience with a younger horse.
What I think I have learned from all of this observing is that a physical response should be a last resort, and that even though I think of myself as patient, I need to be even more patient with a foal. I try my best to stand my ground and ignore any of his antics that are well out of striking distance. When he does threaten my space I square up to him, doing my best impression of "The Look" and only if he is ignoring that do I take a swish at him. And yes, I have begun carrying a "tail" with me. I will use either a bit of thin rope or baling twine looped around to make a nicely swishable tail that is only meant to sting a bit when I give a slap with it. Also, like a dominant horse's tail I never chase him with it, he has to run into it by invading my space to be slapped. So far so good, Z is usually respectful enough of my space that I don't feel threatened, and he is beginning to integrate into my herd like a properly adjusted colt.