It (Pennywise the Dancing Clown)

It (also commonly known as Pennywise) is an ancient alien/eldritch monster and the title character and main antagonist of Stephen King's best selling and award winning 1986 novel. It appears in SMG4 as a form of Gooigi.

History
He kidnaps Luigi, so Mario, SMG4, Toad, and Fishy Boopkins went on a adventure to save him. After they went inside a well, he says that he is Luigi. He then transform into Gooigi and attack the gang.