Psycho Mantis Costume
This easter egg is found in the Halloween Special episode titled, "Halloween Spectacular of Spooky Doom".
In the early scenes of the Halloween episode after Zim begins to freak out over Dib being missing from class, if you look in the seat behind Dib’s desk you’ll notice a kid wearing a gas mask and black gloves. This kids costume is drawn in reference to the infamous Metal Gear Solid character, Psycho Mantis. Mantis is best known for his “psychic abilities” in Metal Gear Solid 1 where he’d read your memory card and comment on the games you played.
It was later confirmed in the DVD commentary for this episode by the shows creator, Jhonen Vasquez that this was in fact a Metal Gear Solid easter egg.