Baby

The infant Metroid, also known as the Hatchling (which was a name used in reference to Samus as well), is best remembered in its Infant Metroid stage from Metroid II: Return of Samus and Super Metroid, though it did not remain in that size.

Metroid II
Samus Aran was ordered by the Galactic Federation to exterminate the remaining Metroids on SR388, so they could no longer be used as weapons by the Space Pirates. She encountered several stages of the creatures on her mission, eventually defeating the Metroid Queen. Soon after, she came across a Metroid egg, the last of the species. It hatched as she approached and the infant followed her back to her ship, carving out obstacles and enemies alike as it orbited around its adopted parent.

Super Metroid
Samus dropped the larvae off at Ceres Space Colony, where it was to be examined by Federation scientists. Not long after departing, the Colony was attacked by Ridley. Samus arrived in time to watch Ridley depart with the creature, and escaped the station in time to avoid its self-destruction. Samus persued the fiend to Zebes where she defeated him. The capsule that encased the infant had been found broken, with the last Metroid nowhere to be found. The Pirates used beta-radiation to create Mochtroids from it.

In Tourian, Samus found dried husks of a Torizo, Zoomer, Ripper, and Skrees in two of the later rooms. She then watched as a Sidehopper met the same fate, at the hands of the Metroid, now much larger than any that she had previously encountered. It then turned on Samus, siphoning her health within an inch of her life before recognizing her. Samus continued on through the base to fight Mother Brain, who transforms into an enormous cyborgian monster, firing Hyper Beams at Samus until the hunter could no longer stand. As the creature prepared the final blast, the Metroid darted in and latched onto the attacker. Leaving the husk of remains, the enormous Metroid displayed a previously unknown ability to heal Samus. At this time, Mother Brain rises from the dead and fires upon the Metroid. After fully healing Samus, it turns its attention to Mother Brain, who fires off her own attack before the Metroid has a chance to latch on. The dieing Metroid disintigrates over Samus, and endows her with the Hyper Beam ability, which she avenges her "child" with. Samus escapes the exploding planet, when Mother Brains death triggers an enormous explosive.

Metroid Fusion
Shortly after, Samus is asked to escort a Biologic Space Laboratories science team on SR388. They planned to study the changes in the ecosystem since the Metroid extinction. During this expedition, Samus is attacked by an X Parasite. The creature causes her to become disoriented, and she crashes her Gunship into the SR388 System Asteroid Belt. (In progress)