Fat Guys are enemies that appear Super Mario World 2: Yoshi's Island.
Super Mario World 2: Yoshi's Island
Fat Guys appear as rather enlarged versions of Shy Guys that appear across Yoshi's Island. They appear primarily in Super Mario World 2: Yoshi's Island. Fat Guys are invulnerable to jumping attacks by Yoshis, but they can be swallowed by them. When this happens, Fat Guys are turned into abnormally large eggs, that have the same effect as POW Blocks when thrown.
Although, actually, this Shy Guy is not named fat guy but goes by the name of Gourmet Guy