I guess they’re actually burros, though, off-hand, I don’t know the difference. If I weren’t sitting in a campground away from all cell service, I would look it up. No Google here, though.
Going from memory, it seems burros were once used in what is now Custer State Park in South Dakota to haul people up one of the larger peaks in the Black Hills. At some point, the owner of the beasts released them for some reason, probably no longer economically viable. The wild burros of Custer State Park are their descendants.
We came across these critters on August 6th on a drive through part of Custer State Park after we had set up camp just south of there in Wind Cave National Park.