Overgrown Rover

Overgrown Rover (育ちすぎたポチ, Sodachisugita Pochi) or Rover for short is a Dragon-level Mysterious Being and a member of the Monster Association. Its role is to guard the Monster Association headquarters to prevent intrusions.

Appearance
Rover is a very large black six-eyed creature. He has fangs and resembles a monstrous-like dog. Although appearing very large in size at first, Rover was speculated by Black Sperm to have been weakened since it has gotten smaller in size after the battle between the Hero Association, the Monster Association, and Garou.

Personality
Rover behaves like a dog. It listens to any order given to it, even if it's from the enemy. Despite being one of the most durable and powerful monsters within the series, it has followed simple commands from the likes of Bang and Saitama to sit.

Monster Association Arc
Overgrown Rover encounters Garou and Busaiku when the two are trying to escape to the Monster Association headquarters. '''The following section contains Webcomic spoilers. You have been warned, manga-only readers.''' Garou and Rover begin to fight, Garou ends up telling Rover to sit and punches Rover into the ground where they were fighting, leading them to fall into the headquarter's basement. Rover later encounters Saitama. Saitama tells Rover to sit and punches Rover into submission. Afterwards, Rover encounters Fubuki, Genos, Bang, and Bomb, who were trying to enter into the headquarters. Rover starts fighting against them and during the battle none of the members of the group are able to injure Rover, even after using their combined attacks. Out of annoyance, Bang rhetorically asks why Rover couldn't simply be quiet and sit down. Rover, remembering being forced into submission by Garou and Saitama after they gave the same command, obediently sat and stopped the fighting. For the remainder of the invasion, Rover's enormous size began to reduce drastically until it became not bigger than an average dog.

Psychic Sisters Arc
After the destruction of the Monster Association, Rover rejoins Black Sperm and together they persuaded Saitama to take them to the Hero Association's headquarters in A-City. Rover and Black Sperm are later entrusted to Butterfly DX, Chain'n'toad, and Forte since they lost a bet with Saitama after Forte finally witnessed Saitama's true power with his own eyes.

Abilities and Powers
As a high-ranked member and a Dragon-level Mysterious Being of the Monster Association, Rover is an extremely powerful being, and quite possibly one of the most powerful Dragon-level mysterious beings to date.

Physical Abilities
Immense Durability: Rover is incredibly durable, being able to withstand hits from Bang, Bomb, Garou, and even Saitama without receiving any or little damage, a very impressive if not nearly impossible feat to accomplish. Rover forced Bang and Bomb to use Cross Fang Dragon Slayer Fist, a technique more powerful than the one used on Elder Centipede.

Immense Speed: Rover is quite fast and able to keep up with S-Class heroes such as Bang and Genos.

Energy Projection: Rover is capable of firing destructive energy bursts in rapid succession from his mouth.

Trivia

 * Rover will keep the same design in the manga as the webcomic.
 * Murata has shown it a while back on a wedding card.
 * Rover is the fourth mysterious being who has survived a punch from Saitama, the other 3 being Boros, Garou, and Evil Natural Water, and unlike Boros and Evil Natural Water, he did not have regeneration abilities that kept him alive, indicating his superior power.