The Arm of Hephaestus was a humanoid automaton seen to be worked on by Leonardo da Vinci in the Leonardo da Vinci's workshop holoprogram. It was named after Hephaestus, god of the forge.
It was designed to swing a hammer, to aid blacksmiths. He used goose grease to improve its clockwork mechanism. (VOY: "Scorpion")
Leonardo is in reality indeed known to have worked on a robot, although not this one.