wheelhorse

noun3

1. •
   (US, old-fashioned) One of a team of horses which is nearest to the wheels of a carriage, as opposed to a leader or forward horse.

   • The off wheel-horse a grey, named Single Peeper, sold for 50 [guineas] […] The near wheel-horse chesnut, named Chance. — 1750
2. •
   (US, figurative) A person who labors heavily for a particular cause, without being concerned about recognition.

   • [F]rom his [Benjamin Ruggles'] well-known habits of industry and constant devotion to the interests of his constituents, he was called "The Wheelhorse of the Senate." — 1866
3. •
   (no longer used, historical) A foot-propelled vehicle; a bicycle.