Michigan J. Frog
From the classic Warner Bros. cartoon, One Froggy Evening (1955), and then much later resurrected as the mascot of the WB TV network. This memorable one-shot character is hidden in a box in a cornerstone of a building being demolished. Discovered by a construction worker, he bursts into toe-tapping, roaring twenties-style song and dance numbers- but only when alone with the worker, frustrating the worker's attempts to get rich off him.
He's only referred to in the cartoon as the singing frog, the "Michigan J. Frog" name came later. Singing voice by Bill Roberts.