A bonus in Custer SP. The wild burros. Their ancestors arrived with the miners. When they didn't find gold, they turned the burros loose. The current residents have a neat trick. One walks out to the center line and stops. The others go from car to car, begging for food. The rangers prefer that you feed them carrots, but they'll eat just about anything. Its been awhile. I'll post pics if I can find them.
BTW, CSP is a state park, so the National Parks Pass doesn't work. But the State pass isn't expensive, and you'll need it to camp.