IDIOMS

willing horse

(idiomatic, dated) One who readily performs hard work or who voluntarily tolerates an adverse situation.

Example: 1863 Nov. 10, "Gen. Sherman's Column," New York Times (retrieved 12 Feb 2014):
  [A]fter a good deal of discussion (some of it angry) among the Major-Generals, it was settled as such things are everywhere—the willing horse (which SHERMAN always is) getting the work to do.
1869, R. D. Blackmore, Lorna Doone: A Romance of Exmoor, ch. 32:
  [S]he said to me as quietly as a maiden might ask one to carry a glove, "Jan Ridd, carr thic thing for me."
  So I carried it for her, without any words; wondering what she was up to next, and whether she had ever heard of being too hard on the willing horse.
1878, Robert Louis Stevenson, An Inland Voyage, ch. 12:
  Finding us easy in our ways, he . . . told me a cock-and-bull story with the moral of another five francs for the narrator. The thing was palpably absurd; but I paid up, and at once dropped all friendliness of manner. . . . He saw in a moment that he had gone too far, and killed a willing horse.
1914, William MacLeod Raine, A Daughter of the Dons, ch. 10:
  "When he hears of it he'll be more anxious than ever to fight."
  Valencia nodded. "A spur to a willing horse."
1999 Feb. 18, Philip D. Delnon, "Education letter: Exhausted, underpaid: and that's a good day," The Independent (UK) (retrieved 12 Feb 2014):
  There is certainly the need to reward performance and offer incentives for success, but flogging a willing horse is not the way to do it.