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Giant Shy Guy are Shy Guys that are fatter and bigger. They make their only appearance as regular enemies in Super Mario Advance and are absent from the original version of the game.
Red and blue variants of Giant Shy Guy.
Giant Shy Guys appeared in early trailer of Yoshi's Story, though they are not present in the final game. They also appear in the Yoshi Demo, where stomping one would turn it into a regular Shy Guy. A giant robotic Shy Guy appears in Yoshi's Island DS. One also appears in the WarioWare: D.I.Y. Showcase microgame Mario Adv.
In the small short that concludes the chapter in volume 26 of Super Mario-Kun, a Giant Shy Guy attacks Mario, Goombario, and Kooper. It then pits Mario against Robirdo. Mario, however, grabs the Giant Shy Guy and throws it against the Robirdo, defeating Robirdo and the Giant Shy Guy.
Yoshi's New Island
Giant Shy Guys are confirmed to appear in Yoshi's New Island.