Mad Piano

The Mad Piano is an enemy in Super Mario 64 and its remake Super Mario 64 DS that only appears in one of the rooms in the mansion in Big Boo's Haunt. Its appearance is akin to that of a popular depiction of a grand piano, but with sharp teeth under its lid and a mouth-like red interior. When Mario approaches near by the Mad Piano, it then chases him around the room, taking three wedges of health when it hits him. The only way to make the Piano stop chasing Mario is to be far away from it enough. Behind its starting location is a Red Coin that Mario must get in order to collect one of the Power Stars in Big Boo's Haunt.

Negative Emotional Aura
Although less severe than Wet-Dry World, the Mad Piano has been suspected to have its own negative emotional aura. It is frequently said by players of Super Mario 64 that the Mad Piano was one of the scariest enemies they know of in the game. Such reasoning can be deduced for a multitude of reasons. One of these reasons could be of the relentlessness of it chasing Mario while clamping its jaw up and down again. Another reason could be that it usually cannot be destroyed by any means necessary in the base game, although some copies of the game are theorized to have a way to kill it. The piano itself also is unassumingly to be normal before it would be activated, so the sudden hostility of the Mad Piano could be a shock towards newcoming players.

One other reason on what could be the root cause of the negative emotional aura regarding the Mad Piano could be its out of place nature in the mansion of Big Boo's Haunt. No other entities seem to be near the area where the Mad Piano is in, it's unassuming nature of being a normal piano at first, and the more "realistic", non-cartoony look to the enemy could provide a negative response to the enemy itself.

Mad Piano Could Be Defeated
In the Super Mario 64 section of Nintendo Power Player's Guide, it has been stated that the Mad Piano could be defeated, albeit without any explanation to what players should do to kill it. However, the base copy of Super Mario 64 has the Mad Piano be impossible to defeat, as any contact Mario has with it makes him take damage. As such, it can be seen as an error in the Player's Guide itself.

One theory on why this "error" was made would be that the copy depicted in the Player's Guide in Super Mario 64 was a beta copy of the game, one in which the Mad Piano could be defeated. The guide itself has a lot of the final elements in the game, so it might be a very late beta of the game that was shown off by accident.

Another theory revolves around the Personalisation A.I in Super Mario 64 having a tie to the Mad Piano being defeated in the Player's Guide. It is possible that the Mad Piano was affected by Enemy Discoloring, with a brown version of a piano being defeatable via a ground pound. The piano depicted in the Player's Guide however is the normal black piano. It could be because Nintendo had requested to use the image of the original black Mad Piano in order to stop awareness of the A.I.

Trivia

 * The possible inspiration behind the Mad Piano could be the Japanese horror movie House, with one of the deaths in the movie involving a person being eaten alive by a piano.
 * In Luigi's Mansion 3, one of the bosses of the game known as Amadeus Wolfgeist possess a piano and uses it similarly to how the Mad Piano behaves. Along with this, a poster of what is suspected to be the Mad Piano can be found in the hotel of the game The Last Resort.