The Metroid Larva is the Metroid baby that hatched right before Samus Aran's eyes. The first thing that the baby had seen was Samus, so it seemed to think that Samus was it's mother. Samus took the baby back to researchers for further examination.
 Metroid II: Return of Samus
In the larva's first appearance, it hatched right before Samus' eyes. After defeating the Queen Metroid and when leaving planet SR388, you encountered the lone Metroid Egg. This egg happened to hatch right when you walked by. Apparantly the baby thought that Samus was it's mother. The baby followed you all the way back to your ship where you took him away and back to researchers.
 Super Metroid
In Super Metroid, the baby appeared at the beginning of the game. Ridley swoops down and abducts the baby. After you progress in the game, the baby appears, now massive in size, and tries to suck the life out of you. However, he spares your life, realizing who the good guys are. In the end of the game, you fight a reencarnated Mother Brain, and she nearly kills you. However, the baby comes by and sacrifices it's life, saving you from Mother Brain. Now, you have all of your energy back, and have the ability to use the strongest beam, the Hyper Beam, to destroy Mother Brain.
 Metroid: Other M
In Metroid: Other M the Metroid Larva was recreated from cell fragments obtained from Samus' suit. The Galactic Federation was trying to create a bio-weapon unit that included Metroids, and the Metroid Larva was the main source of propogating the Metroids again. In the end, the Larva was killed by Adam Malkovich when the Metroid tries to attack Samus'. Then, Adam locks himself in Sector Zero along with the other Metroids, and initiated a self destruct sequence, eliminating the Metroid as well as Sector Zero. This is the last appearance of the Metroid Larva to date, and there isn't much of a chance it will come back, as it was eliminated right before Samus' eyes.