Horror Dungeon

Big Boo's Dungeon, also named Horror Dungeon in some copies, is an anomalous course in Super Mario 64. It's commonly reported to be accessed from either Big Boo's Haunt, Hazy Maze Cave, or a different, related anomalous course. The course itself is usually some sort of ruins or, as the name states, a dungeon. The course commonly reuses music from Hazy Maze Cave or Big Boo's Haunt.

Geography
Big Boo's Dungeon usually looks like a rusted dungeon, although some people reported that it looks like ruins instead. The skybox is reportedly just the default Big Boo's Haunt skybox, if it manifests. Camera glitches are needed to see it, though, because it's normally impossible to see the skybox in this course. The enemies in this course are usually Boos, Mr. Is, and Dry Bones, although Scuttlebugs, a Mad Piano, and, oddly enough, Red Goombas have been reported to appear here as well.

Missions
According to most reports, one of the Missions that can manifest with this course requires the player to kill a Big Mr. I. Another report claimed that there was a Big Boo in this course as well, which, like in Big Boo's Haunt, spawns a star when killed. Some reports also state that there was a golden boulder that releases a star when destroyed, like the ones in Hazy Maze Cave.

Theorizing
It's theorized that Big Boo's Dungeon was an early version of either Big Boo's Haunt or Hazy Maze Cave, due to the similarities between the 3 courses. The fact that "Horror Dungeon" was the internal name for "Hazy Mazy Cave" supports this theory.

Alternatively, it could've been created by the AI as part of Dynamic Level Arrangement, although this is unconfirmed.