From the Super Mario Wiki
Debulls are bull-like creatures that are seen in Yoshi's Island DS. Debulls are listed in the Enemy Museum as land enemies. They follow the Yoshi's movements in the game, allowing the player to easily lure them into chasms. They also have a tail similar to Poochy's tail. One of the few ways to defeat a Debull is to use Baby Bowser's fireballs. They can be stunned by luring them into a wall or post, or hitting them with an Yoshi Egg. They can be stunned longer if Baby Donkey Kong is on Yoshi's back when it gets hit by an egg.