Big Boo's Forest

Big Boo's Forest is supposedly a secret course hidden in Big Boo's Haunt, found in certain personalized copies of Super Mario 64. It contains tons of dark trees in a large forest, as well as a large moon shining above the course, being the only source of light. It also contains a cabin found near the middle of the course.

Analysis
As described, the course contains tall, dark trees that cannot by climbed, with a trail leading the player across the forest. While exploring the forest, Boos will come out and attempt to attack the player, alongside Scuttlebugs, and an exclusive enemy known as Dry Bones, which work almost identically to Dry Bones found in other Mario games, falling apart when jumped on only to reassemble themselves after a few seconds. After a few minutes of following the trail, a large cabin can be found surrounded by trees, alongside torches lighting up a door in the middle. The inside of the cabin is allegedly a large, dark room with a bed, a few chairs, a table, and a few bookshelves, alongside a star.

The offside of the trail is unknown, as players to allegedly played through the course were supposedly 'too afraid to leave the trail.'

The course itself is said to contain very little coins on the trail, though presumably more coins can be found off the trail. It lacks any NPCs, though a single sign can be found outside the cabin.

"Eh he he... Congratulations for exploring this haunted forest, Mario! Enter the cabin to gain your reward, if you dare...."

MISSIONS

 * 1) INSIDE THE ABANDONED CABIN
 * 2) SCALING THE TREE
 * 3) OUT THE CABIN FOR 8 RED COINS (Presumably)
 * 4) Unknown
 * 5) Unknown
 * 6) Unknown
 * 7) 100 COIN STAR (Presumably)

Theorizing
The cabin area in Big Boo's Forest bares a striking resemblance to an unknown area in the Spaceworld '95 demo. Although there are presumably no Boos reported to be in the cabin, the texture for the walls is reported to be nearly the exact same.

Due to this, players have assumed this course was cut quite early into development, though the reason is unknown, many theories have sprouted attempting to explain the reason as to why it was cut. On theory explains that it could've been cut due to how simplistic it is, simply being a trail with few curves around trees. It could've also been cut due to being too scary/dark for Mario.

In some reports of those who encountered Big Boo's Forest, it is accessed by a painting in the courtyard which only appears once.