Wheel-horse

(n.) the hardest working member of a team; the member of an enterprise who bears the greatest burden

Back in the 1700s, originally—and quite literally—a wheel-horse was a workhorse, harnessed between or in front of the wheels of a carriage, wagon, or some equally laborious horse-drawn contraption. Positioned in front of these wheel-horses, standing proud of the device to which they were attached, were the leaders, and although all the horses in the team were compelled to work to the same standard, the leaders were seen as taking less of the burden than the wheel-horses behind them.

By extension, the term wheel-horse has come to be used figuratively since the nineteenth century of anyone who works harder than the others around or associated with them, or else the member of a team or enterprise who bears more than their fair share of the burden or responsibility for it.