Big Boo's Forest

Big Boo's Forest is a supposed course in Super Mario 64. There are several varying forms of Big Boo's Forest. It is usually tied to the Ghoul Medal. It is also tied to the skybox of Big Boo's Haunt, which is a forest despite the cage the course takes place on being on grass.

Theorizing
It is possible that Big Boo's Forest is the unused level 37HORROR2. Big Boo's Haunt could be considered "HORROR1", so if there was a course tied to it, it would likely be called HORROR2.