From the Super Mario Wiki
A Caged Ghost.
Caged Ghosts are close relatives to Dangling Ghosts that grow from drainage grates in the floor. They are gelatinous ghosts found mainly in World 3 in Super Mario World 2: Yoshi's Island. They are not very bright, as they spit green Shy Guys at Yoshi, who may turn them into the ghost's only weakness: Eggs.
There's two different kinds, the ones that are squeezed into sewer tunnels, and the large round mounds. By being hit with eggs, the ghost will shrink, until it's at it's farthest point, then after a few seconds it will grow out again.
