Metroids

Metroids are the titular main antagonists of the Metroid videogame franchise, being an artificially created race of life-draining parasites.

Creation
The Metroids were genetically engineered by the Chozo, a race of bird-like creatures who adopted Samus Aran, to hunt down the X Parasites, another race of life-draining pests. However, all Metroids eventually evolved into a neutral predatory race that killed every living thing in sight.

Metroid/Zero Mission
Making their first appearance in media by being the titular antagonists of the 1986 Metroid game (as well as its remake, Zero Mission), the Metroids were discovered when the Galactic Federation began researching SR388, the planet Metroids were created. The Federation then shipped out the Metroids on the space-craft Space Research Vessel Marina, only for an army of Space Pirates to hijack the ship. Samus Aran, now a grown intergalactic bounty hunter, later wiped out the Space Pirate ranks and the stolen Metroids.

Metroid II: Return of Samus
To ensure that the Metroids do not pose a threat to the rest of the universe, the Federation send two squads of soldiers to SR388 to eradicate the Metroids for good, only for both missions to end in failure. They eventually call on Samus to deal with them as she was the only person able to survive multiple encounters with the creatures. Throughout her mission on SR388, Samus came across and defeated every form of Metroid, including their queen, who she defeated in a final battle. On the way to the surface, a Metroid egg Samus finds hatched, and seeing that the baby Metroid mistook Samus as its mother, she took it along with her.

Super Metroid
Samus dropped off the Baby Metroid at the Ceres Space Colony for it to be examined by Federation scientists, only to return shortly after due to a Space Pirate raid. She arrived in time to find Ridley escaping the colony with the Baby in its capsule. Samus escaped from the Space Colony before it self-destructed and followed Ridley to the Space Pirate headquarters on Zebes. As Samus fought her way through Zebes, the Space Pirates used special rays on the Baby to generate clones for a new army of Metroids. Mochtroids, very weak variations of Metroids, are found in Maridia, and the first attempted clones. Actual clones do not appear in the game until Samus reaches Tourian to protect Mother Brain, including a massive Metroid that turns out to be the Baby in its second stage, recognizing Samus before it drains her life completely. The Baby ultimately sacrificed its life to save Samus from Mother Brain, but since some of Mother Brain's laser beam was absorbed by the Baby when it restored Samus's health, Samus obtained the Hyper Beam and destroyed Mother Brain once again and escaped Zebes before it exploded.

Other M
Metroids were cloned from fragments of the Baby on Samus's suit and propagated in Sector Zero of the BOTTLE SHIP, where the Federation performed multiple tests on them. The first cloned Metroid ultimately grew into a queen, meaning that the Baby itself was coded into becoming a queen. The queen and all of its offspring were nurtured by MB, an Android built with the information and personality of Mother Brain. As Samus approached Sector Zero, she spotted an Infant Metroid and immediately thought of the Baby, but she is shot by Adam to prevent her from going into the Sector, who instead sacrificed his life to detach the Sector from the BOTTLE SHIP, which then exploded (this is because the Metroids of Sector Zero were genetically engineered to have their weakness of ice removed, rendering Samus unable to destroy them). The last Metroid, the cloned queen, was fought and defeated by Samus shortly after. Before MB is able to escape from the BOTTLE SHIP and spread Metroids throughout the universe, she is shot and destroyed by Federation soldiers. There is a theory that Phantoon was the true mastermind of the BOTTLE SHIP sabotage due to his appearance in the post game before the ship was destroyed by the Federation.

Metroid Fusion
The Federation had been using the BSL space station circling around SR388 for a secret Metroid breeding program. Samus eventually arrived in the Restriction Laboratory, where the Metroids were secretly breed. The SA-X, following after Samus, attacked the laboratory, but was overpowered by the cloned Metroids (as the SA-X was an X Parasite, the main food source for Metroids). Samus escaped before the Restriction Zone detached from the BSL station and exploded, killing the SA-X and all but one Metroid that managed to escape from the laboratory. The Metroid rapidly evolved into an Omega Metroid and ambushed Samus after she altered the BSL station to crash into SR388 and easily overpowered her, but was attacked by another SA-X that it easily killed. This allowed Samus to absorb the X Parasite from the SA-X, reclaiming her Ice Beam, and used it to kill the Omega Metroid and escaped from the BSL station.

Infant Metroid
The Infant Metroid is the first stage of a Metroid's life cycle, starting immediately after emerging from an egg.

While seemingly harmless due to their small fangs and their tendency to circle around another organism in curiosity, if hungry the infants will unpredictably strike and never let go until their prey is deprived entirely of their energy. Due to being infants, a cold blast from any source is capable of killing it instantly.

Larval Metroid
The Larval Metroid is the second stage of a Metroid's life cycle and the most common member of the Metroid species, appearing in every game of the Metroid series with the exception of Metroid Prime: Hunters.

Using their iconic jaws, they attach themselves to prey and consume their life force. As seen in Super Metroid, the more life force a Metroid consumes, the larger the Metroid becomes. Once the Metroid consumes enough life force, it is able to evolve into an Alpha Metroid.

Alpha Metroid
The Alpha Metroid is the third stage of a Metroid's life cycle. They appear in Metroid II: Return of Samus and in one of the containers of the Restriction Laboratory in Metroid Fusion. Starting from the Alpha Metroid, Metroids lose their weakness to ice attacks, but their underbelly are weak enough to be damaged by regular missiles.

In Metroid II, Alpha Metroids are the weakest type of Metroid fought. Since they lack jaws, their only attack is to ram into Samus. Only five missiles to their tender underside will take them down. They are encountered fifteen times throughout the game.

Gamma Metroid
The Gamma Metroid is the fourth stage of a Metroid's life cycle. They appear in Metroid II: Return of Samus and in one of the containers of the Restriction Laboratory in Metroid Fusion.

In Metroid II, Gamma Metroids can fire electrical beams from their horns to defect missiles and harm Samus. They can be defeated with ten missiles, assuming it does not defect any. They are the most plentiful of the Metroids in the game, with sixteen encounters.

Zeta Metroid
The Zeta Metroid is the fifth stage of a Metroid's life cycle. They appear in Metroid II: Return of Samus and in one of the containers of the Restriction Laboratory in Metroid Fusion.

In Metroid II, they are smaller than Gamma Metroids, but are fast and very maneuverable. Only by shooting its stomach or back does it receive damage. The Zeta Metroid attacks by shooting a fireball from its mouth and rushing at Samus. Twenty missiles are required to take one down. Only three are encountered in the game, all in Phaze 7.

Omega Metroid
The Omega Metroid is the sixth and final stage of a Metroid's life cycle. These evolved form of parasites appear as enemies in Metroid II: Return of Samus as well as Metroid Fusion.

In Metroid II, Omega Metroids, being the most mature of the race, deal an enormous amount of damage when Samus is attacked by them, and can only be killed once shot in the back (or stomach less effectively). They are only encountered four times in the entire game.

In Metroid Fusion, Samus only encounters one Omega Metroid in her mission on the BSL Space Station, it is a large and unforgettable one at that. In the first encounter, the Metroid easily downs Samus as she fires at it in vain. Rendering Samus nearly crippled with its power, the Omega Metroid comes in for the kill, only to get weakened and distracted by SA-X, an evil parasite clone of Samus. This allows Samus to finish the Metroid off and escape.

Queen Metroid
While the Omega Metroid is the final stage of a Metroid's life cycle, some Metroids, similar to an ant or bee, contain certain genetic coding to mature further into a Queen Metroid, the largest and most powerful member of the Metroid species.