shanks' mare

or shank's mare

Idioms
  1. ride shanks' mare, to go on foot rather than ride; walk: It was such a delightful day that we decided to ride shanks' mare to the fair.
Also called shanks' pony, shank's pony.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018