Pluton

Pluton, King of the Underworld (冥界王プル豚, Mēkaiō 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 grey and red sewn patches. He wears red pants and a shirt with green, yellow, brown and grey patches sewn together over with is a rib cage with his shoulder decorated with skulls.

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.

Trivia

 * 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.