Roger the Potted Ghost

Roger the Potted Ghost (initially just the Potted Ghost) is a boss character from Super Mario World 2: Yoshi's Island and is located in the final castle of World 2. Roger started out as a lifeless flower pot, but was transformed by Kamek's magic into a large ghost named Roger. He's amorphous, but his face resembles Dangling Ghosts and Caged Ghosts.

To defeat him, the Blue Yoshi has to push Roger and his pot over the edge of the platform. This is a difficult process, as two green Shy Guys are pushing Roger in the opposite direction. Throughout the battle, Roger will lunge at the Blue Yoshi, knocking them away. He also shoots blue flames which home in, forcing the blue dinosaur to stop pushing and get out of the way. If the Shy Guys are pushed off the edge before Roger himself, he will be helpless and won't be able to move.

In Yoshi's Island DS, there are enemies called Potted Ghosts. They look much like Roger, only much smaller and less malevolent.

Trivia

 * The blue flames he shoots are based on the hitodama, a phantasmagoric phenomenon in the Japanese folklore which is said to be the souls of the dead.
 * Roger is also the only inanimate object that Kamek transforms into a boss.
 * Some stones are seen when he is pushed off the platform, implying that he falls through the floor.