Metroid

The Metroid is an ectoparasitoidic life form created by the Chozo to combat the X Parasite on SR388. However, due to their ability to collect life energy and feed off of it, Metroids have been harvested for experiments and use as weapons by the Space Pirates.

Origins
On SR388, the X Parasite threatened to destroy the ecosystem, due to the high rate of reproduction. However, the Chozo stepped in to save the planet, and created Metroids. "Metroid" is the Chozo word for "ultimate warrior." The Metroids kept the X Parasite numbers in check, and became the dominant predators of SR388. (Metroid Fusion)

Cultivation and Extinction
The Metroids were first found on SR388 by a Galactic Federation scout ship. However, while en route with a metroid, the scout ship was hijacked by the Space Pirates.

The Space Pirates saw Metroids as a weapon they could use, due to their parasitoidic properties. The Space Pirates captured and cloned many Metroids, with some damage to their own numbers. With the Metroids, the Space Pirates engaged in experimentation, using phazon for mutations.

Other forms of Metroids mutated due to the phazon, such as the Hunter Metroid and the Fission Metroid. Metroids were brought to Zebes, Tallon IV, and Aether, where they were expiramented on and cloned. While on Tallon IV, a life form called the Metroid Prime was found. It was highly dependant on phazon to live, and due to its ability to take in energy and use it to feed, the Space Pirates called it a Metroid, although whether the Metroid Prime is an actual Metroid is unknown. (Metroid Prime).

Seeing the dangers the Metroids posed to the galaxy, the Galactic Federation attempted to destroy them. After two failed attempts by the Federation's army to eradicate the Metroids on their home planet of SR-388, they called on Samus Aran to deal with them, as she had previously been the only person able to survive an encounter with the creatures. Samus destroyed all the Metroids on the planet, including the Queen, save one; a larva metroid that believed Samus was its mother. Samus brought back this larva metroid to Ceres Space Station to be used for research. (Metroid II: Return of Samus)

This didn't last long however. Soon Ridley of the Space Pirates captured the larva Metroid and brought it back to Zebes to be cloned. There, the Space Pirates first unsuccessfully cloned the Metroid, creating Mochtroids. They did manage to clone the Metroids successfully, only to be destroyed by Samus. During Samus's battle with Mother Brain, the Metroid that believed Samus to be her mother gave Samus back life energy during a critical moment, leaving Samus to destroy the Mother Brani. However, the Metroid did not survive the encounter, leaving the Metroids to be presumably wiped out from the Galaxy. It should be noted that the Metroid which saved Samus Aran was massive, giving proof that Metroids come in varied sizes. (Super Metroid)

After effects
Once all the metroids were gone from SR388, the X Parasite could reproduce again, and became a threat to the ecosystem. Samus Aran attempted to fight the parasite, but was unsuccessful. The parasite went inside Samus's body, leaving Samus's body integrated with her suit. Parts of it were surgically removed, but she was dying. there was a solution: a Metroid vaccine. The Metroid vaccine killed the X parasite, and saved Samus's life- again.

The Metroid that Samus brought back from SR388 was cloned on the BSL Research Station orbiting SR388. The Federation mangaged to clone each form of Metroid, up to the Omega Metroid. Whether they managed to clone a Queen, or why, is unknown. The Metroid cloning center was jettisoned into space and self-destructed after the SA-X breached it. (Metroid Fusion)

Biology and Life Cycle
The Metroid has a several-stage lifestyle.

Egg-larva
Metroids start out as Eggs, presumably laid by the Queen. They then hatch into a small Metroid larvae. The larvae is small, and floats in the air.

Metroid
The Metroid has a thick gelatinous membrane, resembling a solid jellyfish, which contains a set of red, raspberry-like nuclei. They have two pairs of mandibles, one for gripping their prey while the other extracts its life energy. Metroids hover in the air, defying gravity by using kinetic energy they store in their body.

Once attached to their prey, they are very difficult to dislodge and will otherwise drain their victim of their life energy completely, killing it. They have an endless appetite and will feed on any and all living creatures they come across, with the single exception of their own species. This immense threat is compounded by their invulnerability to most forms of weaponry. They have only one real weakness: an extreme sensitivity to cold temperatures. Once their membrane is frozen it can be shattered with explosives, such as a small explosive projectile.

When a Metroid takes in energy, it grows in size. Eventually, it will metamorphose to its next form.

Alpha Metroid
The Alpha Metroid is the next stage in the Metroid life cycle. While Alpha Metroids can hover like their previous form, they have four limbs, strange tusks, a horn, and two pairs of eyes.

Gamma Metroid
The Gamma Metroids are a grown version of the Alpha Metroid. The limbs are now longer and more muscular, the carapace is thicker and bigger, and the horns are stronger and thicker. Another pair of eyes is added to the head of the Metroid.

Zeta Metroid
Zeta Metroids are more of a dinosaur-like analog. This form of the Metroid has only a small bit of the blue membrane left from the unmorphed Metroid left over. The Zeta has two muscular legs, two short arms with claws at the ends, and a tail.

Omega Metroid
Besides the Queen Metroid, this is the fullest a Metroid can evolve. The blue membrane, usually on the underbelly on the Metroids, is now concealed. The legs, arms, and tail are now fully developed. Another pair of eyes is added, making a total of four pairs of eyes.

Queen Metroid
The Queen Metroid is so far the only form of Metroid known to be able to reproduce normally. The queen is much bigger than any other form of Metroid, and stands on all fours as opposed to walking on two, like the Omega Metroid. The neck of the Queen can extend to high lengths, to better protect her young. (Metroid II: Return of Samus)