wheel horse

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The horse in a team that follows the leader and is harnessed nearest the front wheels.
• noun A diligent, dependable worker, especially in a political organization.

from The Century Dictionary.

• noun A horse harnessed next to the fore wheel of a vehicle—that is, attached to the pole or shafts—as in a four-in-hand or a tandem; hence, figuratively, a person who bears the brunt, or on whom the burden mostly rests.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a draft horse harnessed behind others and nearest the wheels of a vehicle