The Yo-kai that parodies Slender Man is Slen.
Slen is a deliberately shortened, comic-style name that echoes “Slender,” the familiar shorthand often used for Slender Man. The joke fits Yo-kai Watch’s broader approach: its creatures frequently reinterpret urban legends, pop-culture figures, occupations, foods, and everyday annoyances through puns and exaggerated designs.
Slender Man itself began as an internet horror character created by Eric Knudsen, who posted the original images on the Something Awful forums in 2009. The character’s unnaturally tall, faceless appearance quickly spread through online stories, videos, and games, making him recognizable enough to inspire parody in other media.
The distractors are tempting because they describe the source figure rather than naming the Yo-kai. “Slender” is associated with Slender Man, while “Stretch” and “Tallman” merely suggest his appearance. The canonical Yo-kai name in the English reference is Slen.