Black Ridley

Black Ridley (黒リドリー) is a temporary transformation of Ridley seen in Metroid: Other M.

Description
The ability to transform into Black Ridley has only been demonstrated by Ridley's clone, which was created aboard the BOTTLE SHIP as a bioweapon. Therefore, it is unknown if the original Ridley naturally possesses this ability.

After Ridley takes enough damage to stumble (leaving him open for a Lethal Strike if he is on the ground), he will immediately counter with a new ability: while screeching and flapping his wings, he transforms into Black Ridley. Black Ridley is visually distinguished from normal Ridley because his skin is darkened and his torso glows with a purple aura. While in this state, Black Ridley is completely invulnerable to most of Samus Aran's arsenal, including her Plasma Beam and Missiles.

Immediately after transforming into Black Ridley, he flies up and unleashes at least one fireball shockwave. After this, he continues to fight Samus in the air or on the ground, using all of Ridley's normal attacks.

The Black Ridley effect wears off after about a minute and a half, with Ridley performing the same animation that initiates the transformation. However, Samus can fire a Super Missile at him, instantly reverting Ridley back to normal. He can transform into Black Ridley again later during the battle, and will always do so after a Lethal Strike opportunity.

Super Smash Bros. for Wii U
Ridley's clone appears in Super Smash Bros. for Wii U as a stage boss for the Pyrosphere. After taking a certain amount of damage from fighters (enough to make him collapse for a few seconds), Ridley will shift into his Black Ridley form. Unlike in Other M, this transformation is not an innate ability; Ridley must first fly underneath the main platform, smash one of the generators, and absorb "a certain kind of energy" in order to become Black Ridley. It is possible to interrupt Ridley before he does so, preventing him from transforming into Black Ridley; therefore, Ridley can even be defeated without transforming into Black Ridley.

Similar to his appearance in Other M, Black Ridley has darkened skin and his entire body gives off a purple glow. While in this state, Black Ridley uses more powerful variations of Ridley's normal attacks, including the fireball shockwave and arcing-fireball attack. However, he is still vulnerable to all attacks. Black Ridley can be defeated and counts as a KO in timed battles. Even if he flies away without being defeated, Ridley will show up again later in his normal form.

Despite this form not displaying any cybernetic features whatsoever, it is given the name Meta Ridley in the Super Smash Bros. for Wii U 50-Fact Extravaganza. This is actually an erroneous alteration made by the English translation of the video; in the original Japanese version, this form is more accurately named Black Ridley (黒リドリー) and does not reference his Meta form at all.

Super Smash Bros. for Wii U 50-Fact Extravaganza
"Furthermore, if Ridley consumes a certain kind of energy, he will become the even more vicious Meta Ridley."

Trivia

 * Ridley's default appearance in Metroid: Zero Mission somewhat resembles Black Ridley, with black skin and red wings. However, he lacks the purple aura and is still vulnerable to Samus's Charge Beam and Missiles.
 * The game files for Super Smash Bros. for Wii U contain an unused stock icon for Black Ridley, presumably meant to be displayed on the results screen if a fighter KOs Ridley while in his Black Ridley form; in the game itself, Black Ridley uses the same icon as Ridley. This icon is based upon normal Ridley's appearance as a boss in Adventure Mode: The Subspace Emissary from Super Smash Bros. Brawl (albeit edited to have black skin), rather than either Meta Ridley or the updated Other M Ridley design.