Pint-Sized Slasher

The Pint-Sized Slasher, was a Pre-War 20th century serial killer that wore a creepy clown mask, and used a large chef knife as the signature killing tool.

You are able to pose as him in the mission Tranquility Lane, as you have to kill all residents while posing as the killer child clown himself.

Note: While in the costume, all residents of Tranquility Lane will run away from you in terror.

In Point Lookout add-on, his mask can be found in the room 1K of Homestead Motel. The room is also filled with skeletons.

Article
One of the newspapers shown on loading screens mentions the Pint-Sized Slasher. The full article can be accessed via the Capitol Post newspaper office in L'Enfant Plaza: