Boo

Boo is an enemy in the Super Mario series of games. In Super Mario 64, they appear in the Castle Courtyard and Big Boo's Haunt. Unlike the Mario games before it, here Boos can be easily defeated by Mario by attacking them, however they are immune to being jumped upon.

Boo Key
Perhaps one of Super Mario 64's most infamous unused objects is the Boo Key object, a key that would've resided within a Boo. This key can be seen in pre-release screenshots, and functionality for the key can still be found within Super Mario 64's code. There is even colour variations of the key, and an icon for the HUD only found within the Japanese version of the game (in the European release it is replaced with a Ü, and in the American release it is removed entirely).

The actual use for this key is unknown, as there is no code tying it to unlocking doors or anything similar. The most likely use for this key would've been unlocking doors in Big Boo's Haunt. While the key was removed from the final game, a few recall actually having to collect these keys for the first mission of Big Boo's Haunt.

Boo Hordes
Some personalized copies reportedly spawn Boos in large groups known as Hordes. This usually happens randomly, more commonly in anomalous levels such as Big Boo's Forest or the Secret Coffin, though there are also reports of events triggering Boo Hordes, such as defeating a Big Boo or entering a specific mission.

Theories
The existence of Boos in the Castle Courtyard, aside from being the location where Big Boo's Haunt can be entered, poses questions about their relation to the castle. Some have theorized that the Boos maybe the ghosts of dead Toads, therefore depicting the Castle Courtyard as a graveyard. This could also potentially pose the idea that the Eternal Star statue, infamous for the "L is Real" plaque, could very well be a grave of some kind. If it is a grave and is to be believed to relate to Luigi, then this and the brother's absence from Super Mario 64 could imply that Luigi is dead during the events of Super Mario 64. It's also interesting how that this tie between Luigi and ghosts predates Luigi's Mansion.