Just back from a couple of days camping by the South Warner Wilderness (extreme northeastern California), and thought I would post a few pictures.
There's a nice equestrian campground, East Creek, at the south end of the wilderness. About a dozen camp sites, each with a couple of individual corrals for horses. It doesn't seem to get much use: we were the only people there, other than a couple of hunters who came to the trailhead for the day. (Only problem is that the directions on the Forest Service website are wrong: it says go 3.2 miles east of likely before turning on Blue Lake road, but it's closer to 9 miles.)
Riding is scenic, but a little rough. There are two trails leading into the wilderness. The East Creek trail leaves from the campground, and goes downhill through an old burn & manzanita for a few miles before petering out. The Summit trail starts with a pretty stiff climb, then heads out along a nice ridgeline for miles. The trail is pretty rocky for long stretches, though.
There looks to be a lot of other good places to ride, along the Forest Service roads, and meadows.