E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Wooden Mare (The).
The mare foaled of an acorn. An instrument of torture to enforce military discipline, used in the reign of Charles II. and long after. The horse was made of oak, the back was a sharp ridge, and the four legs were like a high stool. The victim was seated on the ridge, with a firelock fastened to each foot.
Here, Andrews, wrap a cloak round the prisoner, and do not mention his name unless you would have a trot on the wooden horse.Sir Walter Scott: Old Mortality, chap. ix.