ride a horse foaled by an acorn

obsolete To be publicly executed, especially by hanging in the gallows. A nickname for the gallows in Georgian England was the "wooden mare," as they were often made of oak; hence they were "foaled by" (grown from) acorns, the nuts of oak trees. Many a knave has been sent to ride a horse foaled by an acorn for less than the crime of which you have been accused, my young fellow.