noun

• (US, dated) 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."
• (US, figuratively) 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.""
• (obsolete or historical) A foot-propelled vehicle; a bicycle.
  "In 1830 a bold and vigorous attempt was made to utilize the wheel-horse. A French post-office official, M. Dreuze by name, brought forward an improvement on the old two-wheel velocipede, which bid fair to be successful. […] A number of the country letter-carriers were mounted on the wheel-horse, and whilst the roads continued dry and hard M. Dreuze could congratulate himself on the success of his invention; but with wet weather came bad roads, and to the wet succeeded frost and snow. A little extra labour was all that was required to overcome the extra friction of the bad roads, but the wheels refused to progress on the slippy frozen surface."