This piano might look ordinary at first sight, but it isn't an ordinary piano because if Mario, Luigi, or Wario walks near its lair, the piano will show its sharp pointy teeth and chase after one of them and try to attack them, causing them to lose three bars of health. The Mad Piano appears in the first room of the mansion on the bottom left corner and protects a Red Coin found behind it. This enemy is invincible meaning it is impossible to defeat and is best if it is just avoided. The Nintendo Power Player's Guide states that it's possible to defeat the Mad Piano, though the statement is false.
What appears to be the Mad Piano makes a cameo on a poster in the Last Resort hotel in Luigi's Mansion 3. Amadeus Wolfgeist transforms into a Mad Piano when he enters his final phase, which has him possess a piano he was playing.