Well, that聮s an interesting question. But really, how much will you need rain pants in July? Granted, afternoon thunderstorms are a probability, if not a certainty, but the odds that you and your horse will spend days on end trudging through a downpour are remote. One inference from all this is that you聮re not going to want to ride in thunderstorms, anyway. You聮ll see it coming, hop off the horse, don any appropriate rain gear, and ride it out under a tree for the 30 minutes or so it takes to pass.
Mountain Hardwear Backcountry Recon Pants

Still, I do see your point about 聯at least some time in the saddle.” Patagonia聮s Guide Pants ($140; patagonia.com) are nice pants, but they聮re not really a rain pant. They聮re water-resistant, but are aimed more at repelling a little blowing snow or mist than a real drenching.
One option would be to simply buy a pair of fairly inexpensive rain pants with the expectation that this one trip might be their last. If the seat wears a little during the trip, so what? Marmot聮s Full-Zip Precip Pants ($90; marmot) would fit that bill nicely. Mountain Hardwear聮s Backcountry Recon Pants ($195; mountainhardwear), on the other hand, are three-ply and reinforced for durability. So they would keep you dry AND probably survive time in the saddle.
Another solution is to buy rain chaps. Filson聮s Tin Chaps ($60; cabelas.com) are rainproof and very tough. Plus they聮re quite stylin聮. Like other true rain wear, they聮re not going to be the most comfortable thing if the weather is wet AND warm, but chaps are cooler (temp-wise) than full pants.
Have a great trip!
The Gear Guy reports from , the bi-annual gearapalooza in Salt Lake City. Check out his top picks for gear to watch in 2007.