Boohemoth

Boohemoth is a type of Big Boo introduced in New Super Mario Bros. 2. It is possibly the largest Boo in the Mario series, only rivaled by Bigger Boo.

It appears in a specific part of a Ghost House, where it poofs out of nowhere and chases Mario. When looked at, it acts like another Boo, where he closes his eyes and covers it with his arms, blushing red in the process. However, if it is looked at for too long, it'll peek through its arms, and if looked at even longer, will apparently chase after Mario again, regardless of his direction.