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 3 primary objectives of the organization: strike fear to the human population through monster's rampage; kidnapping important hostages that are vital to the operation of the Hero Association and monsterize several powerful human to boost military strength of the organization.

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

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

Monster Raid Arc
Several Members are sent out to take Hero Association members as hostages and rampage around the cities.

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.