From the Super Mario Wiki
Meta Ridley is a transformed version of the dragon-like alien creature Ridley from the Metroid series. He first appears in Metroid Prime, where he is upgraded from his biological state by the Space Pirates. In Super Smash Bros. Brawl, Meta Ridley is fought as a boss a few levels after Ridley is defeated.
 Role in the Subspace Emissary
In the Subspace Emissary, Meta Ridley appears in one of the later levels, after Ridley has been defeated, and he attacks Captain Falcon, Captain Olimar, R.O.B., Samus Aran, Pikachu, Diddy Kong, and Donkey Kong as they try to escape the floating island before it is engulfed in subspace.
As this battle is fought while trying to escape an island, there is a set amount of time that the player has to defeat Meta Ridley, who is more difficult to beat than Ridley was. Additionally, there is the added difficulty of the battle taking place on top of Captain Falcon's ship, the Falcon Flyer, so it is possible for the player to fall off the edge and lose. Meta Ridley boasts some of the same attacks as Ridley did, such as scraping his tail across the top of the Flyer and firing fireballs, though he has many new attacks that involve grabbing the Falcon Flyer and lowering it down with his weight, sometimes even lowering it off of the screen.
Fortunately, most of Meta Ridley's attacks can be avoided by jumping off the Falcon Flyer at the right time. Meta Ridley's fireballs can also be reflected right back at him.
If the player manages to defeat Meta Ridley, a cutscene is shown, and the Falcon Flyer escapes while the island is engulfed into subspace.
Meta Ridley appears again in The Great Maze, where he is battled again in one of the rooms. He is also fought in the Boss Battles game mode.
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