From the Super Mario Wiki
Mock Ups are strange balloons in Super Mario World 2: Yoshi's Island and its remake Yoshi's Island: Super Mario Advance 3 that only appear in a few levels. When a Yoshi gets too close, they inflate. If a Yoshi eats one before it pops, a 1-UP is awarded. However, if the Yoshi fails to eat the balloon, the Mock Up would pop and reveal a Fly Guy, who would fly away mockingly, while the Yoshi is injured.