The Underchomp is an antagonist in some chapters. The Underchomp has three heads, each being able to talk. Yellow is the youngest, Red is the middle one, and Blue is the oldest.
Each of the Chomps have different kinds of abilities: the red one breathes red fire, the blue one breathes blue fire, and the yellow one has a rancid breath.
They have chased the trio through various places before the events of The Adventures of Yoshi, Boo, and Paratroopa. Their first appearance is in Chapter 3, where they chase the trio in the portal realm. During Chapter 4, they fight the trio again. A similar scenario happens in Chapter 7, during a tournament inside the Beanbean underground arena.