Boohemoth

Boohemoth is a type of Big Boo introduced in New Super Mario Bros. 2. It is the largest Boo in the Mario series. Unlike the other members of the Boo species, the Boohemoth has normal human-like incisors instead of fangs, visible cheeks and chin, and it has two eyeholes with a white iris and black pupils. Boohemoth has a very low-pitched laugh compared to its fellow Boos. Its name is a combination of "boo" and "behemoth".

New Super Mario Bros. 2
In New Super Mario Bros. 2, Boohemoth appears only in two different Ghost Houses: and  (which also contains a normal Big Boo). In, Boohemoth appears in the main corridor. In, when Mario or Luigi swim through the flooded hallway, the water level decreases and Boohemoth appears. In these specific areas of the two Ghost Houses, the Boohemoth appears out of nowhere and starts following Mario and Luigi. It controls the movement of the camera. Just like normal Boos, if Mario or Luigi turns to face it, it closes its eyes with its hands and remains motionless along with the camera. However, unlike Boos, if Mario or Luigi stare at the Boohemoth for a while when far away from it, it will peek through its hands and sneak dangerously close to them, being one of the only Boos to not be stopped if looked at.

Due to its huge size, Mario and Luigi cannot go through the Boohemoth, and as such, they will automatically lose a life if they are squished between it and a wall. Also, the Boohemoth cannot be defeated by any of Mario's power-ups (even as Gold Mario, Invincible Mario, or White Raccoon Mario), making it an invincible enemy, so Mario and Luigi can only run from the Boohemoth until they reach the exit.

Mario Kart 8
Though Boohemoth does not physically appear in Mario Kart 8, a statue based on it is seen on top of the entrance of Twisted Mansion.