Pluton

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

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

Personality
Like several of the other kings, he was a greedy, power-hungry tyrant who wanted 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. As he is about to kill a defenseless child, Genos arrives and saves the kid, before introducing 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, as a Dragon-level disaster, it can be assumed that he was very powerful. Pluton seemed 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 possessed colossal strength fitting his size, as 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 had very impressive speed and reflexes. He was able to traverse the distance between City Z and City Y within a short period of time. Although it's unclear how much his large size hampered his abilities.

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

Trivia

 * Pluton is the 6th king of several kings 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.