This article contains various irremovable spoilers.
Read at your own risk!
The Mystery Creature is a carnivorous purple lizard and is the second stage in the evolutionary cycle of the bio-form Ridley. Encountered in the Bottle Ship's Biosphere (Sector 1), it disrupts the Galactic Federation 07th Platoon's analysis of data pertaining to the cultivation of bioweapons. The platoon ventures outside of the Exam Center and engages the creature. As Samus joins them, the creature spies and leaps upon her from the side of the building, pinning her to the ground. It tries to stab her with its razor-sharp tail, and Samus must shoot Missiles at said tail to prevent her untimely demise. Eventually, the creature is sideswiped by a blast from Anthony Higgs' Plasma Gun, which knocks it onto its back, where it writhes in agony. After a few seconds, it regains its footing and spirals through the air and into a wall previously concealed by the hologram that is Sector 1. It is later seen in Sector 3 (the Pyrosphere) as an empty shell suspended above a doorway.
NTSC-U: What once was a cute little bird has changed into a lizard-like creature. It still has some white feathers from its previous form, but that's where the similarities end. Is there something in its blood that drives it to confront Samus whenever possible? Her parents were Ridley's victims, after all.
PAL: If Little Birdie is Ridley's cute baby years, this must be his awkward teenage phase. The feathers on his back are the only real clue that it's the same creature. Is there something in Ridley's genes that make him hate Samus so much? The original Ridley killed her parents, and now this one is out to get her too!