Big Boo

Big Boos are bosses in Super Mario 64 found in Big Boo's Haunt. They are a larger, boss version of Boos. One is found on the balcony accessed from the attic, one is spawned by defeating all Boos on the first floor of the level, and the last one is found by defeating all the Boos in the merry-go-round. The Big Boos act the same as regular Boos, though require 3 hits to defeat instead of 1, and make the usual sound bosses make when damaged. In Super Mario 64 DS, another version of them is present known as King Boo found in the Big Boo Battle level.

Bigger Boo
In some personalized copies of Super Mario 64, Big Boos are larger than normal, or a third tier exists that is even larger than Big Boos. These versions usually require 5-10 hits to defeat, and reportedly can drop the anomalous Ghoul Medal, or a Big Boo's Key.

Big Boo's Key
Big Boo's Key is the name of an unused key collectible found in earlier builds of Super Mario 64. They would be dropped by specific Big Boos, and likely unlock a floor or doors in Big Boo's Haunt, the staircase, or absolutely nothing and simply function as a useless collectible, similarly to the most common manifestation of the Ghoul Medal.

Trivia

 * In Super Mario 64 DS, King Boo is erroneously referred to as a Big Boo, both in dialogue and in the Big Boo Battle level it is found in.