Fan Nickname: The monsters are never directly named in-game (but are so in the game files) so a variety of nicknames sprung up. Grunts, the jawless ones, have gone by Mr. Face (or Mr. Flappyjaw) while Brutes, the metal-infested faceless ones have gone by Mr. Tall. Both "Mr. Face" and "Mr. Tall" were names given to them by Mangaminx. They have also been called "Mister Struts" and "Anti-Struts" respectively, by Spike and Barley.
While we're on that topic, Pewdiepie also refers to the grunts as "Bros".
The Grunt has also been nicknamed Mr. Flappyjaw, and the water monster has been called simply Sewer Monster (most likely because it reminded some of the Sewer monster in Silent Hill 3 ). The water monsters do have a proper name, but it's incredibly easy to miss and hard to spell or pronounce.