Green’s Dictionary of Slang

wooden-legged mare n.

the gallows.

1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn) n.p.: Acorn. You will ride a horse foaled by an acorn, i.e. the gallows, called also the Wooden and Three-legged Mare. You will be hanged.
1811		Lex. Balatronicum.
1890–1904		Farmer & Henley Sl. and Its Analogues.