From the Super Mario Wiki
Killer Chairs are enemies found in Super Mario 64 and its remake, Super Mario 64 DS, in the level Big Boo's Haunt. As their name would imply, they resemble wooden single-person chairs and there are two total in the game. They can be only be found in one room: the second door on the left, when entering the mansion from the front. When Mario enters the room and approaches one of them, the chair rises up into the air, rapidly rotates, comes to a stop and then hurls itself at Mario. If hit, Mario loses two slices of damage. After hitting Mario, the chair will continue flying in its projected direction until hitting the floor or wall and then disappears. If it misses, the same will happen.