Mysterious Beings

Mysterious Beings (怪人, Kaijin) is a term used to refer to sentient creatures that terrorize the cities of the entire planet and pose a threat to society. They have been appearing more frequently in the past 15 years, but humanity still is unaware how such creatures came into existence. It is still unknown what their actual purpose is, but it seems that all these creatures are hostile towards mankind, wreck havoc in the cities and terrorize its citizens.

Mysterious Beings seem to have been normal beings or individuals which through certain means gained superpowers or mutations via scientific or supernatural means while others appear to be sentient races that have been dormant, living in seclusion from the surface world, or even aliens from other planets. Most bizarre are the Mysterious Beings that were normal individuals whose change happened through performing an action obsessively

Mysterious Beings are especially concentrated in Z-City and Q-City. These beings are the reason the Hero Association was founded by Agoni.TBH

Disaster Levels
The attacks by Mysterious Beings have a scale on how destructive the attack is, which goes from Wolf level to God level. Note that the disaster levels are used for all threats to society, not just Mysterious Beings (such as the Giant Meteor).

Mysterious Beings=

Trivia
During the opening of the One Punch Man anime, we actually see the disaster levels of Tiger, Demon, Dragon, and God. (In that order) Also to note is that if you look close enough, you can see the outlines of the names in kanji.

Despite being considered a God level threat by fans, during a Q&A section with the original author, it was stated that Boros is above Dragon. This could mean that Boros is in an unknown level between Dragon and God or indeed a God level threat.

Anime/Manga Difference

 * Wolf is never said or is even considered a disaster level in the anime.