Mock Up is an enemy in Super Mario World 2: Yoshi's Island and Yoshi's Island: Super Mario Advance 3. It's a strange balloon that can be colored red or green.
Super Mario World 2 Yoshi's Island
Mock Up only appears in a few levels. When Yoshi or his allies get too close, they will inflate. If they eat one before they pop, they'll get a 1-Up. If it successfully pops, it'll reveal a Fly Guy.
Red Mock Up (left) and Green Mock Up (right)