From the Super Mario Wiki
Boohemoth is a type of Big Boo that is introduced in New Super Mario Bros. 2. It is one of the largest Boos 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 its eyes look different. Its name is a pun on Behemoth, a large, fabled beast in fantasy culture.
 New Super Mario Bros. 2
In New Super Mario Bros. 2, Boohemoth appears only in two different Ghost Houses: World 2- and World Flower-. In certain areas of a Ghost House, the Boohemoth will pop out of nowhere and chase Mario and/or Luigi. It controls the movement of the camera. Just like normal Boos, if Mario or Luigi looks at it, it closes its eyes with its hands and remains still along with the camera. However, unlike Boos, if Mario or Luigi stare at it for a while when far away from it, it will peek through its hands and slowly sneak up to them up to a dangerously close distance.
Due to its enormous size, Mario and Luigi can't go through the Boohemoth, and getting crushed between it and an obstacle will result in losing a life. Also, the Boohemoth is completely invulnerable, even to Gold Mario's fireballs, the Starman, and the Invincibility Leaf, making it an invincible enemy.