wheel horse

American Heritage® Dictionary of the English Language, Fourth Edition

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

Century Dictionary and Cyclopedia

1. n. 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.

GNU Webster's 1913

1. n. one of the horses nearest to the wheels, as opposed to a leader, or forward horse; -- called also wheeler.

WordNet 3.0

1. n. a draft horse harnessed behind others and nearest the wheels of a vehicle