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Biting Bullet Bills are unused enemies in the SNES game Super Mario World 2: Yoshi's Island. These red Bullet Bills come from similarly-colored Bill Blasters, and they dynamically chase after Yoshi while their mouth makes a two-frame chomping motion. It slows down over time until it ceases to function, quickly shrinking in blue and vanishing in a generic pop. They can be defeated by simply stomping or swallowing them; it looks like a regular Bullet Bill if spat back out, but it will fly downwards. In the Game Boy Advance port, Yoshi's Island: Super Mario Advance 3, they are used and make their only appearance in the level Endless World of Yoshis.
While these specific Bullet Bills have yet to reappear in the series, this concept was reused for subsequent appearances of Bull's-Eye Bill, which originally only switched their direction once.