Burros, miles apart. The dark ones are from the town of Beatty Nevada, Where they can roam thru the town. The grey ones are also there, but are of a divergent strain. They seem to keep to their stock/family tree, and do not mingle. The dark ones are of a slightly taller and stocky build.
PS....This is right near the ghost town of Ryolite NV. You can come around a corner, and find them on the road.