Here is the song "I'm the Guy," a comic song of 1912, with lyrics by the one and only Rube Goldberg and music by Bert Grant. Or, as the sheet music credits read:
Ravings by Rube Goldberg and Noise by Bert Grant
Billy Murray sings "I'm the Guy," a saying popularized by Rube, so far as I can tell, in comc strip form.
Then there were the "I'm the Guy" pinback (small) buttons. There were a lot of these small comedy buttons, although none are photographed in any detail on the ol' Web so far as I can see. Each one would have a goofy looking guy (mostly drawn by Rube, but not always), usually forking his own thumb towards his own mug, declaring that he, himself, indeed, is The Guy.
These sold well, or so I am told.
Here is the song with a lovely bit of video courtesy of jxhensley.
Related: Ron Evry has sheet music scans and an MP3 here.
Related: Randall Whitaker cigarette/tobacco pinback buttons page.
Sheet music cover from the Lester S. Levy Collection of Sheet Music.