Shanks’ (or Shanks’s) mare, pony, etc.: one’s own legs as a means of conveyance.
Oxford English Dictionary, s.v. “’Shanks’ mare’ in shank (n.), sense 1.b,” June 2024, https://doi.org/10.1093/OED/1175946221.
The part of the leg from the knee to the ankle being the shank, if I use my own legs in place of a horse (possibly one I cannot afford) then I am travelling by shanks’ mare.