Pluton

Pluton is the king of the Underworld.

Appearance
Pluton is a gigantic pig-like creature and appears at the very end of the twelve episode.

Alien Conquerors Arc
He wrecks havoc in Y-City. However Saitama and Genos quickly come and dispose of him.

Abilities & Powers
Not much was seen of his abilities, due to how quickly Saitama disposed of him, however it can be assumed that he is very powerful because his threat is categorized at a level of "dragon"

Physical Abilities
Immense Strength: Pluton easily destroys a large portion of Y-City.