Waterwraith

The Waterwraith (アメボウズ, Amebouzu) is a mysterious lifeform discovered by both Captain Olimar and Louie within PNF-404 deep underground in an area becoming known as the "Submerged Castle". It's similar to the Goolix, Plasm Wraith, and Leech Hydroe in a number of aspects, but unknown if they are related in any way.

Biology
What few reports expeditions had made on this strange being, many of which are left in vague detail due to the nature of the Waterwraith. It has a transparent, Humanoid-like body. It has a head, arms, legs, and torso but no precise details (facial features, sexual organs, etc.) are visible or even evident on its anatomy. Every report describes it on top of two stone rollers, which it uses similarly to that of an Earth steamroller, and just as deadly.

While the rollers are solid and clearly made up of a stone-like substance, the actual body of the Waterwraith (despite its name) is not exactly water. In fact, the body of this being cannot be sensed by technological scanners or sensors, indicating that the actual body is anchored to another dimension entirely.

While nearly invisible most of the time, Purple Pikmin are able to turn it into a vulnerable state due to the thudding sounds they create when thrown. This turns its body into a more solid (and actually touchable) substance, if only for a brief moment.

Behavior
Unlike most other lifeforms on PNF-404, the Waterwraith has been shown a few signs of intelligence during its encounters. These emotions aren't shown actively while it has powers, but it shows signs of fear in the sound of Purple Pikmin, even more so when its rollers are destroyed. Very few who dare go this far report that it turns extremely paranoid in this state, often running in a complete panic before vanishing. This confirms that it is smart enough to recognize that these individuals are a threat to it, and one report even describes it as losing all its confidence without its weaponry to back it up.

Abilities
A rather ominous feature this creature has on explorers is its ability of fear. All reports describe a unnatural level of fear and hysteria whenever an encounter with the Waterwraith occurs, to the point of near panic and even insanity. According to some of Louie's reports, leg spasms and internal thunderings are also caused by the Waterwraith during encounters.