Boohemoth

Boohemoth is a type of Big Boo introduced in New Super Mario Bros. 2. It is possibly one of the largest Boos in the Mario series.

It appears in a specific part of a Ghost House, where it will appear out of nowhere and chase Mario or Luigi. Just like normal Boos, if Mario or Luigi faces looks at it, it closes its eyes with its hands and remains still. However, unlike Boos, if Mario or Luigi stare at it for a while, it will peek through its hands and soon begin to chase after Mario or Luigi instead of staying still for an infinite amount of time.

Trivia

 * The name is a play on the words "Boo" and "behemoth", which means a great monster.