Metroid

Metroids are a deadly race of space-dwelling parasitic organisms that originate from Nintendo's Metroid video game series. They are a regular threat to the bounty hunter Samus Aran and have been deemed dangerous enough for the Galactic Federation to target them for extermination. They feed on the life energy of other creatures to sustain themselves and have proven difficult to kill.

History
The Metroids were originally a genetically-engineered organism created by the ancient Chozo race as a means of destroying the deadly X Parasite. They were successful in fighting off the X and drove it to near extinction, but the creatures proved to be too difficult to control and the Chozo were forced to abandon their colony on SR388. The Metroids left on SR388 quickly rose to the top of the planet's food chain.

The Metroids would one day be discovered by the Galactic Federation who captured a number of specimens for scientific research, but the frigate carrying the creatures was attacked by a Space Pirate raiding party. The Space Pirates discovered the Metroids and saw that they possessed significant military potential. Since this incident, the Pirates have focused much of their efforts into using Metroid genes as material for developing biological weapons or using Metroid specimens as organic batteries. Because Metroids are capable of absorbing energy so efficiently, the Space Pirates would often try to use them as living batteries to power machinery on several of their bases.

Because of the Space Pirates' interference, Metroids have spread beyond the boundaries of SR388 and have appeared on a number of other planets across the galaxy. The Galactic Federation have deemed the Metroid infestation a serious threat to galactic security and have called for a galaxy-wide eradication of all Metroids. The bounty hunter Samus Aran has been regularly called upon by the GF to investigate Metroid hot-spots and to destroy any parasites she encounters. Her mission to hunt down Metroids has taken her to many worlds, including SR388, Zebes, Tallon IV and Aether. Metroids have proven to be quite adaptable and have grown in different ways depending on the environments of the planets they've been discovered on.

Metroid life cycle

 * Juvenile Metroid - The most commonly encountered variety of Metroid (pictured above). At their smallest, they are roughly the size of a basketball. Their bodies consist mostly of a translucent, gelatinous membrane through which their sensory organs can be seen. They constantly float through the air and will quickly fly towards potential prey, latching on to their victims with their large mandibles and using a pair of smaller fangs to drain a victim's life energy. It is unknown precisely how this energy-siphoning ability works and, stranger still, the Metroid's fangs don't leave any visible wounds. The more energy a Metroid consumes, the larger its body will become. Since they consume energy, Metroids possess a high resistance to energy-based weaponry. They are vulnerable to extreme cold, however, and Samus Aran has killed many by freezing them with an Ice Beam and then shattering their frozen bodies with concussive missiles.
 * Alpha Metroid - The next phase in the life cycle of Metroids from SR388. When Juvenile Metroids grow large enough, they molt and emerge from their juvenile carcasses in this larger, more vicious form. They do not latch on to prey but rather repeatedly ram at them, essentially beating victims into submission. They are still vulnerable to cold-based weaponry and can be shattered with missiles when frozen.
 * Gamma Metroid - When an Alpha Metroid advances to its Gamma phase, its body becomes larger and heavier. It develops the ability to strike prey with arcs of electricity. It is vulnerable to missile strikes to its head and back.
 * Zeta Metroid - When Metroids molt from their Gamma stage, they become wily bipedal creatures that walk on the ground. They attack by charging prey or by shooting them with organic projectiles. It takes Samus about twenty missiles to kill one of these creatures.
 * Omega Metroid - A hulking beast of a Metroid after growing beyond its Zeta phase. It is quite powerful and attacks by ramming, slashing with its claws and firing energy bursts. Samus has only encountered five of these creatures before, four on SR388 that were protecting a Queen Metroid and another years later on the Biologic Space Laboratories Research Station. Like all preceding Metroid types, the Omega type is vulnerable to the Ice Beam followed by missiles.
 * Queen Metroid - Omega Metroids of a superior genome are capable of evolving into Queens which, like insect queens, serve the purpose of propogating the Metroid species. Samus faced one such Queen on SR388, which fought back by ramming at Samus and extending its neck to bite her. It could also release bursts of energy and would frequently spawn more Metroid Juveniles to defend her.

Mutant variants
Numerous Metroid variants have been encountered on different worlds. The Space Pirates have also conducted numerous experiments on the creatures in order to create the perfect living weapons. The different Metroid sub-types are listed below.
 * Mochtroid
 * Tallon Metroid
 * Hunter Metroid
 * Fission Metroid
 * Dark Tallon Metroid
 * Phazon Metroid
 * Hopping Metroid
 * Metroid Hatcher
 * Unfreezable Metroid
 * Metroid Prime