To fide the wooden horse was a military punishment formerly in use. This horse consisted of two or more planks about eight feet long, fixed together so as to form a sharp ridge or angle, which answered to the body of the horse. It was supported by four posts, about six feet long, for legs. A head, neck, and tail, rudely cut in wood, were added, which completed the appearance of a horse. On this sharp ridge delinquents were mounted, with their hands tied behind them; and to steady them (as it was said), and lest the horse should kick them off, one or more firelocks were tied to each leg. In this situation they were sometimes condemned to sit an hour or two; but at length it having been found to injure the soldiers materially, and sometimes to rupture them, it was left off about the time of the accession of King George I. A wooden horse was standing in the Parade at Portsmouth as late as the year 1750.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Wooden Spoon * Wooden RuffNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes