Plane Mario

Plane Mario is a one-time enemy in Super Luigi Country and appears in Level 9. He is a form of Mario that is playable in Super Mario Land for the Game Boy.

Story
This isn't the first time you see Mario in-game, he was the first boss, Dr. Mario. It is still unknown why Mario is going against his own brother, and it is unknown if he is really siding with Bowser on this.

Level 9
In this level, Plane Mario will fly vertically on the far left shooting bullets while you try to clime up the platforms.

Trivia

 * Mario is the only boss to show back up later on in the game.