Omega Metroid

The Omega Metroid is the sixth and final stage of a typical Metroid's life cycle. The Omega Metroid is is about twice the height of a human with exoskeleton that leaves only a small area of soft membrane at the ribcage exposed. Like the previous Zeta stage, the Omega Metroid has a leech-like mouth, bipedal structure, a tail, and clawed limbs.

Samus Aran encountered the Omega Metroids during her mission to SR388. She would later fight another Omega Metroid created by Galactic Federation experiments on the Biologic Space Laboratories research station. It is never fully explained as to why the Omega Metroid is in the docking bay while the BSL Space Station is being propelled towards SR388, however it is likely that it escaped from the Restricted Laboratory before it was jettisoned from the main space station and developed. Indeed, several Metroid shells can be located around Sector 1 (SRX) after the incident, each one being left behind as the Omega Metroid metamorphosized.

Samus was not initially powerful enough to defeat the Metroid. However, with the assistance of the SA-X and its Ice Beam ability, the Omega Metroid was defeated.

Manual
"This mutation is absolutely huge; its power, incredible!"

Inconsistencies
The Omega Metroids differ significantly in structure and behavior between Metroid II and Metroid Fusion. In Metroid II, the Omega Metroids are only about 150% the size of Samus, while in Metroid Fusion they are in fact much larger. However, this may be because screen size and resolution differ for the Game Boy and Game Boy Advance. Also, the Metroid II Omega Metroids are able to levitate, spit plasma energy spheres from their mouths, take damage from normal Missiles, and were immune to the Ice Beam. The Fusion Omega Metroid is not able to levitate, does not fire any projectile, is immune to Ice Missiles (and just about everything else), and is only damaged by the Ice Beam. The developers of Fusion purposely changed its specifications. The loss of flight in the Main Deck may be the result of being in a different state of maturity, the increased body mass, the airless environment, intended or unintended imperfections in the Federation breeding process (perhaps making it vulnerable to the Ice Beam would make containment easier), or the area that it mutated in factoring into its physical characteristics. Bearing in mind that the breeding environment was filled with X-Parasites, absorbing them could improve some of its traits like they did to Samus. However, there is no evidence to suggest that these theories actually account for any inconsistencies between the two games.

Metroid II
The Omega Metroids are quite a bit slower than their Zeta counterparts, but can deal an enormous amount of damage. They maintain the basic structure of their last forms, and at the same time keep the same weaknesses. Samus must shoot them either in the stomach or back, although a shot to the latter causes more damage.

Omega Metroids typically levitate around while periodically pausing for a moment. Samus can use the Screw Attack to counter a lunge attack from an Omega, and to preserve her health. Omegas also spit deadly acid in battle. Without utilizing their weakness, they require 40 missiles to bring down.

Samus Aran encountered four Omega Metroids during her mission to SR388.

Metroid Fusion
Samus only encounters one Omega Metroid on the BSL research station. This specimen is much more powerful than the Omegas encountered on SR388, able to reduce Samus' health to a critical level in one swipe. After discovering the Restricted Lab, Samus will start to find discarded membranes, likely signs of the Omega Metroid evolving, since the Omega Metroid started off as a 2nd stage Metroid.

Samus enters the room, hoping to make a swift escape of the Station. Instead she finds a giant shell, and her ship nowhere in sight. Samus begins to run back, but suddenly the Omega Metroid bursts through the wall! Samus shoots it in vain, but none of her current weapons even seem to damage it at all. The creature slashes her just once, throwing Samus to the other end of the room. As it moves in for the kill, the SA-X joins the fight. The SA-X ignores Samus completely, and moves right up to the Omega, shooting the creature's chest with the Ice Beam, revealing the Omega's weak point to Samus. The SA-X is also hit by the Omega's claws, and the X form Samus had seen previously floats formlessly in the air. Samus uses up the last of her strength to leap into the X, restoring all her health and returning the Ice Beam to her. This scene is a recreation of the ending of Super Metroid, where Samus is saved from death by a former attacker and is given a powerful weapon to use against her enemy. The Omega Metroid slowly creeps toward the left side of the docking bay throughout the duration of the battle in an attempt to swipe Samus with its claw. Samus uses the Ice Beam to drive the creature backwards in order to avoid this (This battle's mechanics are slightly similar to the battle with Crocomire in Super Metroid). If the creature manages to swipe Samus with its claw if she wanders too close, Samus will be knocked to her knees to the end of the docking bay exactly as she was at the beginning of the battle. Samus succeeds in driving the creature back to the wall through which it came, and eventually the beast is destroyed. Samus's ship returns to the bay, allowing her to leave the station with moments to spare.

Trivia

 * For unexplained reasons, the Omega Metroid attacked Samus despite the latter having Metroid DNA, whereas the other Metroids in the Restricted Area ignored her. This may be because the Metroids in the Restricted Area were preoccupied with the SA-X. (Not to mention the fact that the SA-X approached the Restricted Area and started to attack the Metroids) One of the Metroids (which transformed into the Omega Metroid) probably took Samus as the SA-X, due to the similar appearance. Or, it just may be a generally violent creature, and therefore attacked Samus. Or perhaps it saw Samus as competition, as there was no Queen Metroid aboard.
 * The Omega Metroid is the only fully organic boss in Metroid Fusion not infected by the X-Parasites, due to being a Metroid and is the X's natural enemy and also having an immunity to infection.
 * The Omega Metroid in Metroid Fusion drools some substance after killing the SA-X. This may likely be the acid it spat out in Metroid II.
 * The Omega Metroid appears in the Metroid II commercial, towards the end. It is the only enemy that is not a drawing in comic book style.