Agree with what was said - I'm an amateur photographer and have occassionally done horses for friends. I would also say, head to a horse show and practice. See what angles work and what doesn't. Also - for jumping/movement shots, if you have a nice camera you should be able to set it to take multiple photos in a row, that way you should get at least one good one.
And take TONS. That way you are more likely to have usable photos.
"The white horse moved like a dancer, which is not surprising: a horse is a beautiful animal, but it is perhaps most remarkable because it moves as if it always hears music."