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Nightmare Fuel: New Super Mario Bros. 2
There is nothing okay about the Boohemoth, the giant Boo in the ghost house in World 2 and World Flower. It has a terrifying face, being between it and a wall means instant death, and it doesn't even play by Boo rules- if you stare directly at it, it will occasionally peek out from behind its ghost-hands and inch forward a bit.
What really gets you is when you're just entering the hall it's in. It just appears out of nowhere with a high pitched laugh that almost gives you a heart attack. Huh. An empty hallwa- HOLY CRAP!
The game introduces a Pirhana Plant equivalent of Dry Bones. Its appearance is unsettling at best.◊