Walker Pass has some helpful amenities (including, when I was there this weekend, a huge "donut cache" of pastries and other sweets kindly provided by trail angels for hungry PCT hikers!), but they aren't always easy to access for equestrians. There are three small but well-make metal corrals as you first approach the Walker Pass campground on the PCT, along with a picnic table and a large more-or-less flat open area with some grass, at least at this early time of the year.
The problem is water, which is not available in the campground itself, but there is a water trough and spring just north and west of the campground. To find it, you can go out to Highway 178 and turn left, follow the road for a couple of hundred yards, then walk down into a ravine on the left to locate the water trough and spring. Or, as a perhaps easier route, look for a trail heading toward the highway that comes off of the lower part of the parking loop (near the junction where the loop connects with the entrance driveway).
Head over the hill in the direction of the highway (toward the trees at the top left of the photo). The trail will fade away and become indistinct the further you go, but as long as you head down toward the highway and keep to the left as you go, you will descend into a shallow ravine. If you come across the wire fence near the bottom of the ravine, just follow it to the right toward the highway. You will see a large square concrete trough full of cattails and a longer, narrower water trough. Two years ago the trough was full of water; this year it had only a shallow layer along the bottom, but the large one with the cattails was full and had an overflow pipe with a steady stream of water falling from it.
Long narrow trough (with only shallow water at this time).