We trailer camp but we have friends who pack horse camp. Those would be much different lists as gunslinger said. With either one, your horse needs should be the first list to make. Check out where you are camping, some state and national parks require certified weed free hay be the only kind you bring in. We high line ours using a couple of old cinches as tree saver wraps to tie them between. Some parks don't allow any tying to trees so that's another thing you need to check. In that case you can either hobble, use portable corrals or just put them in the trailer for the night. Some people prefer to bring their own water that their horse is used to drinking with them. That means getting a water tank big enough to hold the amount they will need for your trip. Personally, I'm not that particular and just take enough for the road trip.
There's an excellent book by Don West, Have Saddle Will Travel: Low Impact Trail Riding and Horse Camping. I see it used on ebay
once in awhile. Another good one is Horses, Hitches and Rocky Trails by Joe Back