Biting Bullet Bill

Biting Bullet Bills are unused enemies in the game Super Mario World 2: Yoshi's Island. It did make a later appearance in the Endless World of Yoshis in GBA port, they are a sub-species of Bullet Bills that only appear in one level. They are dark red (their Bill Blasters are also this color). These Bullet Bills, as their name suggests, attack by biting. A bit like Missile Bills, they chase after Yoshi. If they hang around too long, it'll disappear. Biting Bullet Bills can be defeated by simply swallowing them or jumping on them. When Yoshi spits this enemy back out, it looks like a regular Bullet Bill, but it flies downwards.

Trivia

 * Fully functional code and sprites of the Biting Bullet Bill exist in the original SNES version of Yoshi's Island, despite the enemy being unused.