The Tin Man was a character from the Earth story The Wizard of Oz. He was a mechanical man made from tin who longed to be Human. His lamentation of not being Human was expressed in the tune If I Only Had a Heart. By the end of the story, he learned that he was always Human. He did not realize this as he worried so much. (TNG: "The Schizoid Man")
For the lifeform nicknamed "Tin Man", please see Gomtuu.