The Metroid hatchling is a baby Metroid that plays a significant role in the Metroid series. It first appeared at the end of Metroid II: Return of Samus and has been seen or referenced in some form in each of the games set after its conclusion.
Metroid II: Return of Samus
After defeating the Metroid Queen, Samus finds a lone egg that hatches just after she arrives. Since Samus is the first creature that the baby Metroid sees upon being born, it mistakenly imprints her as its mother.
Not willing to kill the hatchling, the last known Metroid in existence, Samus takes it with her as she leaves SR388 and presents it to the scientists at the Ceres Space Colony. Samus leaves the baby in their care and departs, but isn't gone for long when she receives a distress call indicating that the colony is under attack. She returns to find the scientists all dead, and the hatchling stolen by her nemesis Ridley.
After returning to Zebes, Samus explores the labyrinthine corridors of the rebuilt Space Pirate hideout in search of the baby. Late in the game, she is attacked by a giant Metroid that devours her energy until it comes to a realization and flees. The giant Metroid was in fact the baby, exposed to massive amounts of radiation by the Space Pirates, and once it realized that it was attacking its "mother," it retreated.
At the climax of Super Metroid, Samus is nearly defeated in battle with the revived Mother Brain when the baby dashes in and latches itself to Mother Brain's head. After draining a significant amount of energy from Mother Brain, it shields Samus and infuses her with the energy it had collected. However, Mother Brain kills the Metroid soon after it heals Samus. This sacrifice has an impact on Samus that is a key point in Metroid: Other M.
Metroid: Other M
In Metroid: Other M, it is revealed that a rogue group of scientists used sample's of the baby's DNA taken from Samus's power suit in order clone and breed new Metroids for military intent. All of the cloned Metroids on board the Bottle Ship are thought to be destroyed at the end of the game.
In Metroid Fusion, the baby Metroid is able to save Samus's life one more time, if indirectly. After she becomes infected with the X Parasite, Galactic Federation doctors are able to cure her using a vaccine created from remnant samples of the baby Metroid's DNA. However, as a result of this, Samus is now part Metroid, to an extent, and shares the weaknesses of the species, such as their vulnerability to cold.