Proteus Ridley

Proteus Ridley is the unexpected final boss of Metroid: Samus Returns.

Background
Following Samus's numerous defeats of Meta and Omega Ridley in Metroid Prime 3: Corruption, Ridley managed to regenerate enough of his original body to no longer require at most half of his cybernetics.

As Samus is preparing to leave SR388 with the baby, a newly hatched Infant Metroid that imprinted onto her, Proteus Ridley unexpectedly arrives and proceeds to capture the Metroid. In the first phase of the battle, Ridley clutches the baby in his talon as he attacks Samus.

First phase
Ridley begins the battle in the air. His attacks include breathing purple fire balls either in an arc of five, or individual streams. He will also attempt to jab Samus with his tail, and quickly slam onto the ground before taking flight again. Ridley's attacks are very powerful, with many taking out a full Energy Tank. His weak point is his chest and mouth area, which Samus can take advantage of using Super Missiles, Plasma Beam shots or the Beam Burst. Regular Missiles are useless against him.

After taking enough damage, Ridley will recoil and release plenty of Aeion Orbs to refill Samus's Gauge. He then attempts to fly away, but Samus grabs his tail. Ridley attempts to shake her off, but Samus blasts into his chest with her thrusters and pumps Plasma Beam shots into his body. Ridley screams out in pain and eventually drops the baby, which Samus jumps at and grabs as she releases from Ridley. He attempts to charge at her, but Samus shoots at his wings and sends him into the cliffside. He is buried under rocks, but quickly rises up and engages Samus again.

Second phase
In the second phase, the baby levitates in the background near Samus's Gunship. Ridley will begin to focus on more ground-based attacks, including swipes with his claws and breathing individual blasts of fire. He takes flight if Samus jumps over him, and his tail will now cause the ground to release spikes after he jams it into the ground. Ridley also begins to swoop in and grab Samus, slamming her into a wall several times and then throwing her aside. This attack can kill Samus easily, unless she equips the Lightning Armor.

Once he has taken more damage, more Aeion Orbs will be released from Ridley. Ridley shields himself against Samus's attacks, promptly throwing her at a wall and pinning her down. Before he can kill her at point-blank range, the baby intervenes and leeches some energy from Ridley, but is quickly shaken off. The baby gives the energy to Samus, which restores part of her health (equivalent to 2 full energy tanks) before the final phase.

Final phase
Ridley's attacks become more desperate as Samus continues to battle him with assistance from the baby, who steals energy from Ridley for Samus twice during the fight. He uses the same attacks, but now rapidly sprays fire and shoots a blast directly at the ground, causing a shockwave. He will also now combine his tail jabbing and swooping attacks.

After the baby drains some energy from Ridley, he will launch himself at Samus, which she can reflect with her Melee Counter. This begins a sequence where she blasts Ridley in the chest, and then climbs up his body to fire Missiles or Beam shots down his throat. Ultimately, Samus and the baby are both able to subdue him, and leave SR388.

However, Ridley was apparently able to revive himself, and remove the rest of his prosthetics. He then pursued Samus and the baby to Ceres Space Colony, where Samus would donate the baby for research. Ridley would rampage the station, abduct the baby and bring it to Zebes, setting Super Metroid in motion.

Some time following the battle, a Hornoad found Ridley's abandoned mechanical claw and attempted to gnaw on it, before it caught sight of a more interesting, gelatinous yellow being.

Trivia

 * Owing to him being a variation of Meta Ridley, several of Proteus Ridley's vocal sound effects are recycled from his Meta form, as well as his mouth and chest weak points.
 * Ridley was not originally present in Metroid II: Return of Samus, of which Samus Returns is a remake.
 * Samus Returns marks the first time that Ridley has been the final boss of a Metroid game; while Zero Mission featured the Ridley Robot as its final boss, it was merely a robot built in his likeness.
 * His presence in the game was first hinted at in the Overview Trailer, where Proteus Ridley's theme was heard during the Aeion gameplay portion. A glimpse of his hand was also seen in the September 13, 2017 Nintendo Direct, two days before the game's release, with the narrator also hinting at his identity by saying "Vengeance has a name".
 * There are several scenes in the Proteus Ridley fight that reference previous games:
 * The scenes where the baby drains energy from Ridley to give to Samus references the climax of Super Metroid, in which it completely drains Mother Brain of energy and gives it to Samus along with the Hyper Beam.
 * The scene where Ridley pins down Samus only to be saved by the baby references confrontation with the Mystery Creature, who is actually an adolescent form of Ridley, in Metroid: Other M, where the creature pins down Samus only to be saved by Anthony Higgs.
 * The scene where Samus mounts Ridley and shoots directly into his mouth following a successful Melee Counter references the battle with Meta Ridley in Metroid Prime 3: Corruption, where she mounts his head and fires Power Beam shots into his mouth.
 * If Samus dies at any point during the battle, she will have to restart it from the beginning.
 * Proteus Ridley is one of the most difficult bosses in the game; on Fusion Mode his slashing attack is enough to take three Energy Tanks off of Samus's overall health.
 * Proteus, in Greek mythology, is the god of rivers and oceanic bodies of water. He was known for his adaptability and versatility. The Proteus name is appropriate, as it refers to Ridley's ability to easily recover from his battle wounds.
 * Proteus Ridley's theme is different for each phase of the battle. The first is a remix that combines notes from Vs. Meta Ridley, the version from the Prime Series, and Nemesis Ridley, the version from Metroid: Other M (heard here). In the second phase, his theme has an edgier tone with a more tense opening similar to Ridley's themes in Metroid: Zero Mission and Super Metroid (heard here). The final phase retains the tension but with a slower tempo, similar to that of Neo-Ridley's theme from Metroid Fusion (heard here).