Queen Metroid

The Queen Metroid is the final form of a Metroid. One appears as the main antagonist and final boss of Metroid II: Return of Samus, and in Metroid: Other M as the penultimate boss (third to last if counting Phantoon).

Biology
The Queen shares the structure to a crocodile, as she has a long snout filled with sharp teeth and crawls on all four legs. She has four blue eyes on each side of her head, armored platings on her back, and a translucent belly. Similarly to other Metroids, her weak points are her head, mouth, and her stomach, which can be damaged by Bombs or destroyed by a Power Bomb from the inside. Much like an ant or bee colony, the Queen Metroid reproduces the entire species, and only a few Metroids are born with the specified genetic coding to eventually mutate into a Queen.

Metroid II
During Samus Aran's mission on SR388 to eradicate all of the Metroids to prevent them from endangering the universe, the Queen Metroid was the last Metroid left for Samus to terminate, located in the deepest confine of the cavern. After destroying her, Samus witnesses the birth of the last Metroid egg, but kept it with her as she left the cavern.

Metroid: Other M
During the events of Metroid: Other M, Metroids were cloned from fragments of the Baby Metroid on Samus's suit after it was killed by Mother Brain in Super Metroid, and propagated in Sector Zero of the BOTTLE SHIP. The first cloned Metroid matured into a Queen, meaning that the Baby was actually an infant queen. The Queen, along with all the other Metroids, were nurtured by MB, an Android built with the information and personality of Mother Brain. She killed Ridley's clone when it ventured too close to her chamber and absorbed some of its powers, including Ridley's signature fire breath.

Samus comes across Room MW, and finds Madeline Berman in the next room through a glass window. Madeline mistakes Samus as an agent sent to assassinate her and in a panic, opens the Queen's chamber. When Samus ventures inside, she finds hatched Metroid eggs, and immediately realizes the presence of the Queen Metroid as the latter appears right in front of her and roars.

The Queen deploys six Metroids (first one, then two, and last three) from small holes on her back and slams on the ground when Samus freezes the Metroids with her Ice Beam. Once all six of the Metroids are destroyed, the Queen grows crystalline spike platings on her head and neck and sprays a flammable breath at Samus. However, she seems impossible to beat as she grows a second planting once all of the first platings are destroyed. Samus, remembering the kind words of Adam and Anthony, uses Concentration and prepares to fire at the charging Queen, but is interrupted when the ship's emergency brakes activate, throwing them both onto the ground. Samus takes the opportunity to shoot at the Queen's unprotected belly, and quickly enters the Queen's mouth via the grapple beam as she charges a powerful variant of her fire breath. Samus then allows herself to the use of Power Bombs to create a big explosion, killing the Queen.

Trivia

 * As the Metroid franchise was known to have inspiration from the Alien franchise, the Queen Metroid highly resembles the Xenomorph Queen. In both Aliens and Metroid II, the final battle is between the queens and the lead heroines (Samus Aran and Ellen Ripley) and in Alien Resurrection and Other M, the queen is created via cloning.
 * In Other M, the Queen's vision is blurred pink in the cutscenes where she kills Ridley's clone and spots Samus in her chamber. Also, a faint greenish glow can be seen around Ridley and Samus during these same cutscenes, presumably suggesting that Metroids, or at least the Queen, can sense their prey's life force.
 * In Other M, If Samus is unable to fire enough missiles at the Queen's belly or grapple beam her way into the Queen's mouth in time, the Queen either bites or fires an unavoidable fire beam, both killing Samus instantly. She, Vorash, and Goyagma are the only bosses in the game capable of instantly killing Samus.