In "Weirdmageddon Part 1," The Horrifying Sweaty One-Armed Monstrosity encounters Dipper as he attempts to break in to the Gravity Falls Mall in search of his friends. He demands that Dipper climb into his mouth, which Dipper ignores in terror, much to the monstrosity's frustration. He later appears when Wendy and Dipper are trying to avoid Gideon and his henchmen, where he eats one of the Discount Auto Mart Warriors. He also appears during the episode's end credits, still desperately trying to convince anyone to climb in his mouth. He finally gives up and leaves the screen announcing that he needs to call his mother.
He consists of a gigantic man's head, with a round nose, a five o'-clock shadow, thick red eyebrows, and red hair on his temples. Attached to the top of his head is a muscular arm which he uses to drag himself around.
"Anybody wanna get in my mouth? I'm a big mo—hungry monster—I'm a big hungry monster here! I would really appreciate it, if someone—you, ma'am, you ma'am, how would you like to get in my—"
- Louis C.K., the creature's voice actor, improvised most of his role during the end credit scene of "Weirdmageddon Part 1" where he says, "I gotta call my mother." In fact, there is nearly 30 minutes of him improvising different ways the monstrosity could beg someone to get into his mouth.