Plasm Wraith

Strange massive "Mysterious Life-Form" made out of golden goo that appears playful yet overly protective. The "wraith's" appearance depicts a tall and fat figure made out of an unknown golden substance and a single gaping hole in it's head that serves as a mouth

According to the wiki: The Plasm Wraith (アメニュウドウ?, lit.: "Candy/Rain Nyuudou") is the final boss in Pikmin 3 and is responsible for capturing Captain Olimar. It is a virtually featureless and vaguely-humanoid Waterwraith-like being made entirely out of an unknown gelatinous substance, which allows it to split different parts of its body off to create other semi-independent entities or even change shapes.

The Wraith is encountered at the Formidable Oak, having taken Olimar captive with what appears to be curiosity rather than maliciousness. Efforts to rescue Olimar cause the being to enter an alternate form and slowly crawl towards the explorers, attempting to retake him, until it reaches the outside of the oak. Once this occurs, the Wraith assumes its original form, detains Olimar in its nucleus and begins to attack, starting the final boss fight. Defeating the Wraith immediately activates the game's ending.

As with all final bosses thus far, the path leading up to it and/or the boss fight itself requires the knowledge and use of every single Pikmin type.

When Olimar first arrived at the Formidable Oak, he saw a shiny golden substance at the top of the tower, and decided to investigate. Louie expressed discomfort and chose to stay behind, letting Olimar go on ahead without him.

At first, the Plasm Wraith was curious and playful towards Olimar and did no harm to him. However, when Olimar tried to leave, it became hostile, triggering his suit's auto-sleep function and dragging him back to its cave. It became protective of Olimar to the point that it would attack anything that showed interest in him. The exact reason for this bizarre behavior remains unclear.