- Queen Hive
This curious baby Metroid emerges from the last egg after destroying the Queen. Samus initially considers killing it, but seeing as it is the last of its species, and clearly not hostile, decides to let it live. The Metroid then helps Samus escape through Queen's chamber and find her way back to her ship.
After discovering the Baby, it will follow Samus around. It "eats" blocks that are in your way allowing you to progress and return back to your ship in order to escape planet SR388.
The decision to abandon her mission and not destroy the last Metroid on the planet becomes the major plot hinge for the next three games in the series. Super Metroid, Metroid Other M, and Metroid Fusion are all tied to this last surviving Metroid, which you first encounter at the end of Metroid II.