The Bowser Room

The Bowser Room is an area speculated to exist in Super Mario 64. It is generally described as a gray, tiled room with a slope leading to a panel looking out of the room to Bowser. However, some reports claim the Bowser Room's panel is actually a painting. The image was first made by the youtuber OneyPlays, and a clip of them mentioning it can be found here. It is claimed to be Shigeru Miyamoto's favorite room. Some people claim it is part of Princess Peach's Castle, others state it is a hidden easter egg, and others claim it is part of a bad ending.

Possible 64 DS Appearance
There is an image of an unused gray room with Bowser with all 4 main playable characters of Super Mario 64 DS. It is believed that this is possibly a new version of the Bowser Room. This room is also associated with unlocking Waluigi, AKA the ˥ Is Real 2401 theory. It is currently unknown where this room would have been found, though some theories state it is inside the Black Box.

Resident Of The Bowser Room
There is an entity that is reported to be found in the original Bowser Room known as the Resident of the Bowser Room. It is described as a tall black figure. It is advised not to approach it, as unexplainable mental trauma has manifested in those who approach the Resident of the Bowser Room. When avoided, it will disappear after a varying amount of time, from 5 minutes to an hour.