The bodies of automobiles manufactured before 1935 were made from a combination of new metal and horse hoof. There was a great surplus of horses because the car made them obsolete. Thrifty automakers took advantage of this buyer's market and used the hooves, shoes and all, to created a sturdy alloy that was more pliant than steel alone. This practice died out by the early 1940s when large numbers of horses were drafted into the military.

Source : Mino, Paulo. The Horseless Carriage, 1961.