Mystery Creature

Officially known as the Mystery Creature, this lizard-like being is an adolescent form of Ridley in Metroid: Other M. It appears as a large purple lizard-like creature with white fur sprouting from its back.

Encounters
Samus sees the creature attacking the 07th Platoon in the Biosphere Test Area from the Exam Center and runs to save them. However, when Samus arrives, the creature seems to be waiting specifically for her, hiding out of Samus' view on the wall of the building above the door she comes through, and ambushes her. It pins Samus down and attacks her with its tail and sharp teeth. The soldiers try to help her, but they are distracted by swarms of enemies including Reos and Griptians. Samus attacks the lizard's tail as it attempts to strike her head. Eventually Anthony shoots his plasma weapon at the creature, wounding and knocking it off Samus and causing the creature to go into convulsions for a few seconds. Anthony attempts to shoot at it again, but it escapes by crashing through the wall which was disguised by the holographic scene.

The 07th Platoon finds Lyle Smithsonian's body ripped into shreds. After following a trail of green ooze, she finds the husk of a little white creature she encountered earlier, which implies that it killed and ate Lyle's body, helping him along in his molting.

Samus is given orders to chase the monster, but finds only a trail of green blood. Adam instructs Samus to follow the creature to Sector 3, the Pyrosphere, though is shortly scheduled to hunt for Bottle Ships survivors in the Cryosphere, leaving the creature alone for now. Unfortunately during Samus' time in the arctic area, the beast's cries are causing the other creatures to become extremely aggressive.

Much later in the game, Samus sees the creature as an empty husk stuck to a wall above a door, its back ripped open from the inside. Samus continues through the room, where she thinks that she will encounter the next stage in the creature's life. Samus meets up with Anthony Higgs again, where he is trying to fight an unseen flying creature. Samus then sees the creature and realizes that the purple monster was Ridley all along.

The fact that the creature was actually Ridley in an early stage explains why it immediately tried to attack Samus when she emerged instead of attacking the soldiers, and ambushed her when it could have been fighting the soldiers. MB later confirms that the Little Birdie and, consequently, the Mystery Creature were infant forms of Ridley.

Trivia

 * The fact that the station's bioweapons (at the time under MB's control) arrive to defend the Mystery Creature from Adam's soldiers indicates that Ridley was at that time either allied with the android or under her direct control as well.
 * The Mystery Creature may also possess its own abilities to control other life forms, as evidenced by Adam's claim that its roar "is causing the other creatures to act even more aggressive".
 * The creature's tail and back-growths heavily resemble the Xenomorphs from Alien, a series which inspired Metroid. This makes it one of a number of creatures in the Metroid series bearing some resemblance to Xenomorphs (ie: Berserker Lords). Ridley's original appearance in Metroid also heavily resembled a Xenomorph.
 * The way that Little Birdie and this creature "molted" is also very similar to the way the Xenomorph transfers to the next stage of life. Facehuggers die after they have completed their part and live on as Chestbursters, which erupt from the body of a host. Similarly, the outwards bent rib-cages on both early forms of Ridley also suggest the creature emerged in this way, but from the back rather than the chest.