Elmo the Koopa Troopa is a character coming from the Nintendo Adventure Books. He made his debut appearance in Monster Mix-Up. His boss, King Bowser, often calls Elmo the stupidest Koopa Troopa. The Koopa King summons Elmo as part of his plan to fuse him with the Mushroom King, King Toadstool. However, Elmo fails to show up. Boom Boom then appears and states that his fortress guarding duty was getting on his nerves, so he left the fortress and left Elmo in charge. This angers the Koopa King to the point of grabbing the unfortunate Boom Boom Koopa and throwing HIM into the Monster Mixer with a Fire Snake. Boom Boom pops from the crazy contraption seemingly unharmed only for his head to burst on fire. He then runs around for some water as Bowser teases him.
However, the Koopa King DOES manage to fuse Elmo with the Mushroom King, creating an entity that can be called King Troopa. This entity has a Koopa Troopa's body and the Mushroom King's head. The King Troopa helps his liberators by solving puzzles, though Mario does not trust him due to Elmo being used to create him. Later, the King Troopa is split back to the Mushroom King and Elmo, as are the other amalgamations.