Mysterious Beings/Monster Association

The Monster Association (怪人協会, Kaijin Kyōkai) is a Mysterious Beings group led by Gyoro Gyoro and Monster King Orochi. Not much is known about it, except that they are the antithesis of the Hero Association.

History
The Monster Association is an unknown group first introduced to Garou by two members of the said organization. They tried to invite him but are turned down, despite being unlikely friendly.

It's leader Orochi defeated the martial artist Gouketsu and recruited him as a member.

There are three primary objectives of the organization:
 * 1) Strike fear to the human population through monster's rampage.
 * 2) Kidnapping important hostages that are vital to the operation of the Hero Association.
 * 3) Transform several powerful humans to boost military strength of the organization.

After Saitama defeats Garou, the organization collapses. Psykos is taken prisoner.

Monster Raid Arc
In S-City, Human Bird Grass and Junior Centipede were first members to attacked a sushi restaurant where the Hero Associations sponsor Narinki and his son Waganma were at. Metal Bat, who was guarding them, easily defeated both monsters, however Rafflesidon and Senior Centipede came erupted from the ground. Both Narinki and Waganma were knocked out by Rafflesidon's gas and prepare to fight Metal Bat. Metal Bat fights the monsters, but was slightly sedated by the gases the monster released. Metal Bat, however managed to break out of its control by smashing his head and killed both monsters with one strike each.

Moments later, Elder Centipede, a Dragon-level threat, emerged through the ground. Elder Centipede is after Narinki and avoids Metal Bat's attacks. As Elder Centipede was about to catch the sponsor, he was struck by Metal Bat and tries to shake Metal Bat off by maneuvering around the city. Elder Centipede managed to launch Metal Bat far away and continues to rampage through the city. Just as Elder Centipede cornered the heroes Pineapple and Mohican, who were carrying the Narinki and Waganma, it was attacked by Metal Knight missiles. Phoenix Man, Sludge Jellyfish and Rhino Wrestler then corner the heroes and demand the sponsor. After Garou finished his fight with Metal Bat, Phoenix Man and Sludge Jellyfish discuss how they could use him to their advantage.

After Metal Bat was knocked out by Zenko, Sludge Jellyfish tries to take her as hostage, but was struck by Garou. The monster was able to regenerate itself and Phoenix Man attempts to invite Garou into the Monster Association. Garou rudely declines by ripping up the invitation card. The monster say they'll overlook Garou's rude behavior this time and promises that they will meet again, if Garou continues to hero hunt. Meanwhile Rhino Wrestler fights and defeats Pineapple and Mohican and takes Waganma as hostage. The monsters then retreat after explaining how fighting Metal Knight would only be in vain for them. Phoenix Man instructed a retreat to Elder Centipede, and him along with Rhino Wrestler were taken by the large Monster and burrowed underground, but Metal Knight managed to stick to the monster.

The Monster Association has attacked many cities simultaneously:
 * W-City: Heavy Kong has been struggling against Marshall Gorilla.
 * D-City: Lightning Genji fight Maiko Plasma and Electric Catfish Man.
 * I-City: Hundred-Eyes Octopus causes havoc
 * Y-City: Eyesight causes havoc.
 * V-City: Pureblood causes havoc.
 * Unknown city: Do-S causes havoc.

Fist Fight Djinn fight A-Class hero Smile Man, when Bang arrived to the scene. Fist Fight Djinn attempt to attack Bang, but was mercilessly killed by the S-Class hero. Meanwhile, Do-S managed to defeat many heroes and was then confronted by Fubuki and her Blizzard Group. Marshal Gorilla managed to defeat Heavy Kong and tells the civilians to spread the word about him. Maiko Plasma and Electric Catfish Man are still fighting Lightning Genji. Hundred-Eyes Octopus kept devouring building and easily managed to defeat Death Gatling, Butterfly DX and Bone. Eysight managed to poison Ecolo G and other heroes and is fighting Studless. Pureblood managed to kill many civilians.

Many monster that appeared in C-City were defeated by Genos and Face Ripper later encountered the hero. Face Ripper managed to damage Genos, but was caught off guard and killed. In the same city, Awakened Cockroach defeated many heroes and meets Genos in an alley. On the TV, it was reported that Lightning Genji was defeated by the monsters, while Awakened Cockroach managed to have an upper-hand in the fight against Genos, but was caught in an adhesive trap, but managed to escape before being struck by Genos' blast.

Do-S managed to transform all of the Blizzard Group members into her love slave. While Fubuki was preoccupied by the love slaves, Do-S managed to pierce through her telekinesis and whip Fubuki. Do-S manages to hit Fubuki multiple times and believing that she has Fubuki's mind under her control, she reveled her plan of using Fubuki to lure Tatsumaki out. However, Do-S was sent flying and attacked by Fubuki. Angered she decided to kill Fubuki. When the area began to rumble, a monster falls in front of Do-S ans warns her about Tatsumaki. Frightened, Do-S orders her love slaves to distract Tatsumaki, while she escapes.

Super Fight Arc
Gouketsu and the Three Crows hold the Super Fight stadium hostage and recruits 5 members who are promptly killed. Eyesight is eaten by Pig God. Hundred Eyes Octopus is killed by Tatsumaki. Nyan transforms several Smelly Lid Prisoners into monsters.

Members
Dragons= Demons= Tigers= Unknown=

Webcomic and Manga Differences

 * The Monster Association is expanded upon in the manga, having more members and having them come out to the public.
 * In the original web-comic, it only had 17 members: Black Sperm, Bug God, Devil Long Hair, Evil Natural Water, Fuhrer Ugly, Gale and Hellfire, Goddess Glasses, Gums, Homeless Emperor, Overgrown Rover, Phoenix Man, Psykos, Pureblood, Royal Ripper, Senior Centipede and The Great Food Tub.
 * They also did not have Monster Cells in the webcomic.