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 dog like creature. Its eyes are ruby colored. It has very large fangs and four toes for each paw. Its fur is very roughed up. It seemingly lacks a tongue. '''The following section contains Webcomic spoilers. You have been warned, manga-only readers.'''

After the battle between the Hero Association, the Monster Association, and Garou, Rover was speculated by Black Sperm to have been weakened since it has gotten smaller in size.

Personality
Rover behaves like a dog. It is a diligent and loyal guard dog, instantly tracking down Garou after the Hero Hunter helped Tareo escape and did not stop under any circumstances. However, under intense and traumatic conditioning, it was forced to learn to sit even if the command was given by an enemy.

Rover, however, does not instantly grow hostile toward intruders, as he neither attacked nor followed Garou and Tareo in his initial encounter with the pair, leaving them alone when they backed off slowly. Additionally, Rover does not seem to differentiate from friend or foe when angered, as he attacked and killed fellow monster Unihorn after the latter accidentally hit Rover during a fight with Garou.

Monster Association Arc
Overgrown Rover encounters Garou and Busaiku when the two are trying to escape to the Monster Association headquarters.

After Garou and Busaiku slowly walk away from Overgrown Rover, three other monsters, Super Mouse, Showerhead and Unihorn appear and fight Garou. Unihorn uses his "Trinity Demon Bursting Speed Thrust" and launches itself at Garou. Garou brushes him off, and he shoots into the distance, which Garou takes a moment to realise that that was where Overgrown Rover was, and hears its roar. Unihorn's head is then thrown back. After waiting for some time, Rover finally joins in the battle and fires a burst of energy from his mouth. Garou grabs Busaiku and narrowly outruns the energy blast. Once Garou makes it to the surface, he tells Busaiku to run while he handles Rover. Garou strikes Rover's front left leg in an attempt to throw him off balance, but Rover merely shrugs it off and fires another energy blast. This time, Garou is unable to escape it, and is forced to take the full brunt of the blast, which destroys several floors of the Monster Association headquarters in the process. When Rover sees Garou isn't defeated yet, he fires a rapid barrage of energy blasts, except this time, Garou manages to avoid all of the attacks, and strikes a powerful blow on Rover's forehead.

However, Rover merely shakes off the attack and comes out completely unharmed, shocking Garou. Rover then grabs Garou's leg with his teeth, and shakes him around like a ragdoll before shooting another point blank energy blast at Garou, which destroys several more floors of the headquarters. Afterwards, Rover stops pursuing Garou.

'''The following section contains Webcomic spoilers. You have been warned, manga-only readers.'''

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, Overgrown Rover is an extremely powerful being, and quite possibly one of the most powerful Dragon-level mysterious beings to date. During his battle with Garou, he was able to effortlessly tank several blows from him and held the advantage for the entire match.

Supernatural Abilities
Energy Projection: Rover is capable of firing destructive energy bursts in rapid succession from his mouth. These energy blasts are powerful enough to incinerate stone, level city blocks, and destroy several floors of the Monster Association Headquarters with ease along with severely injuring Garou and killing two Demon-level beings in a single shot. Rover's energy blasts can also cause shockwaves strong enough to shake the surrounding surface area of Z-City that Saitama mistook as an earthquake.

Physical Abilities
Immense Strength: As a Dragon-level monster, Rover boasts a tremendous amount of strength. He was able to slam Garou against the wall and floor with ease, and break a monster that boasted possessing a nigh-unbreakable horn.

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 also 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. He was also able to outpace Garou, and was fast enough to run horizontally across the wall of the Monster Association Headquarters.

Fighting Style
Guard Dog: Due to the canine nature of Rover, he is assigned to guard the entrances of the Monster Assocication against intruders despite being a Dragon-level monster and an executive of the association. Whenever one enters his domain with hostility, Rover immediately and relentlessly attacks them to the extent of completely destroying them, seemingly giving up only when the target is dead or escapes.

Trivia

 * Murata has shown Rover's design on a wedding card before the monster's manga debut.
 * 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.
 * In the webcomic, 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.