I've seen quite a few "horse camps" in National Forests here in the Pacific Northwest. Most are dry-camping only, of course, but do usually have water (at least for the horses).
Washington State Parks partnering with local horsefolks recently opened a State Park near Cle Elum that's dedicated to horses. Drove through last time I was over there and though still looking a little raw, it's got all the facilities one could hope for if traveling with horses. I think there are twenty or so hookup sites, free drycampng, and I saw a pretty nice shower house that one doesn't even need coins to use.
More info here.