The Mad Piano is an average looking piano from Super Mario 64. However, when someone goes near the instrument, it will open and close its lid, revealing its hidden teeth. The Mad Piano will then attack the trespasser.
It is possible that the Mad Piano is related to the Chain Chomp, considering its shiny black exterior, sharp teeth, red mouth and their general tactics of attacking anything that comes too close to it.
You can just close the piano if you want, or open it up, like how it’s in the picture. The chair is also in the download link. Enjoy!
By Supermariofreak, info from Mario Wiki