Pluton

Pluton, King of the Underworld (冥界王プル豚, Meikaiō Puruton) was the king of the Underworld.

Appearance
Pluton is a gigantic pig-like creature with red eyes and tubes running from the side of his nose and neck to his shoulders. His skin is dark with gray and red sewn patches. He wears red pants and a shirt with green, yellow, brown and gray patches sewn together over with is a rib cage with his shoulder decorated with skulls.

Personality
Like several of the other kings, he is a greedy, power-hungry tyrant who wants to take the surface-world for himself, enjoying the deaths of several of the humans he slaughtered and hating them for living in gluttony and littering the world with worthless objects.

Alien Conquerors Arc
Awakened due to constant pollution on Earth, Pluton appeared in Y-City and wrecks havoc in said city. Before he was about kill a defenseless child, Genos arrived and saved the kid before he introduced himself to Pluton, buying some time for Saitama to appear and eliminate the monster, but the latter eventually cries out and curses the heavens as his life returns to one-punch defeats.

Abilities and Powers
Not much was seen of his abilities, however it can be assumed that he is very powerful because his threat is categorized as a Dragon Level threat. Pluton seems to have the ability to suck surrounding debris with his powerful nostrils, assuming that is how he destroyed a large portion of Y-City.

Physical Abilities
Immense Strength: Pluton possesses colossal strength fitting his size, he was able to easily destroy a large portion of Y-City.

Immense Speed and Reflexes: As a Dragon-level monster, it can be presumed he has very impressive speed and reflexes, although it's unclear how much his large size hampers his abilities. He was able to traverse the distance between City Z and City Y within a short period of time.

Powerful Nostrils: Pluton has very large and powerful nostrils that he uses to suck people and buildings into his body with, and they are capable of tearing up large portions of the city.

Trivia

 * Pluton is the 6th king of several king mentioned in the series: the Subterranean King, the Beast King, the Deep Sea King, the Ancient King, the Sky King, the Monster King, and the Forest King. He is the 1st king to be given a name.
 * The Kanji Ton in its name is the Kanonyomi for "Pig" (豚 Buta), referring to the monster's animal motif.
 * The name "Pluton" is a reference to the Roman god Pluto, ruler of the underworld.
 * Murata's sketch of Pluton.