Before you look at any of them read Think Harmony with Horses by Ray Hunt and True Unity by Tom Dorrance. Then take a look at Brian Nuebert and Buck Brannaman they have spent a lot of time with Ray Hunt and Tom Dorrance and have the best grasp of the why with the ability to demonstrate and teach the how as well.
This is some of the best advice you could get. (Just don't be put off by Tom Dorrance's book, it'll mean more to you as you go on with this, and it's meant to be re-read, for the attitude of it. )
I've also appreciated the work of Josh Nichol and Harry Whitney - not sure if they were in the orbit of the great Ray Hunt and Tom Dorrance; but like those two great masters, and like Buck Brannaman and Brian Neubert (sp?), they start from observing horse behaviour. So whatever they offer or ask - from their very first approach, even the way they feed - it'll make sense to the horse, as a horse. I think that's "natural" horsemanship.
Recently a friend discovered, and recco'd to me, a clinician named Cynthia Royal, who is based in southern California. I've been following her approach, and it's made all the difference in my leadership (I mean my leadership as judged by my horse, the one who counts). I started a thread about her on this forum, with the website address where you'd find out more about her, videos, etc. The Royals work with some powerful stallions and whatnot, but of course the leadership skills are basically the same - and just as needed, really - for a pony.
Good luck and have fun.