The Underchomp is a boss found in Chapter 7 of Super Paper Mario.
The Underchomp is a Chain Chomp with three heads, each having a different color (red, yellow and blue).
The Underchomp guards the Underwhere, serving as an obstacle to anyone who get to Dorguy the Third, who is also the one responsible of summoning him as a test to Mario and his team. Unlike other bosses in the game, the Underchomp is fought in a Dragon Quest battle style. Each of its heads has a special move, the red head can breath fire, the blue head breathes blue fire and the yellow head has a stinky breath unlike an elemental attack like the red and blue ones. Once all of them are defeated, Mario, Luigi and Bowser can pass by Dorguy.