Mules were used underground in the mines for hauling. They were not only strong, but intelligent as well:
"In an area too low for the mule to stand upright, the animal would go down on its knees to get through. On steep inclines, they learned to push the cars with their chests. Pursued by a runaway car, a mule would duck into the first recess in the wall and let the runaway go clattering by. And the mule knew when it was quitting time." (from "Boss Whistle" by Lynn Bowen, pg. 20).

This leather helmet was used as head protection for mules working in the mines.