Mysterious Beings

Mysterious Beings (怪人, Kaijin), or monsters, are non-human creatures that pose a threat to society.

Overview
According to Dr. Genus, Humans can naturally become monsters from a desire to transform into something else through their bad habits, a complex, or a burst of dissatisfaction with their regular selves. These factors trigger abnormal cell reactions that prompt their transformation. Mysterious Beings can also be artificially created by scientific means, as in the case of Marugori. Non-human life can also be transformed into Mysterious Beings through factors such as pollutants, environmental changes, and genetic engineering. Some Mysterious Beings belong to non-human races and are considered natural adversaries of mankind. For example, Pureblood came from a bloodline of ancient vampires; Boros was born an alien.

The biological mechanisms of some monsters are similar to humans; their metabolic processes require oxygen intake and carbon dioxide waste emission. Other monsters operate by the use of inexplicable methods. For example, the only organs Evil Natural Water possesses are its eyes and heart, so how it controls its liquid body is a mystery.

Generally, when a human transforms into a monster through excessive compulsive behavior, their resultant transformation draws influence from their obsession. For example, Crablante overate crab and became a crab monster; Devil Long Hair's obsession with hair gave his monsterized form the ability to manipulate his hair. On the other hand, transformation via monster cell ingestion amplifies the strength and talents of the user. The drawback of this monsterification method is the loss of the ability to develop further strength. According to Choze, who ate a monster cell, a drug is secreted in the brain which removes the morals of the user. Mysticism may also be a mean of transformation into a monster, as in the case of Sword Devil Executioner, who sold his soul to his blade and turned into a monster.

The frequency in monster appearances has increased in the past 15 years. Reasons for hostility against humans vary. Monsters like Piggy Bancon and Tongue Stretcher are simply public nuisances. Creatures like Crablante and Surprise Attack Plum attack to exact revenge. Clans of creatures like the Seafolk and the Subterranean People seek possession of the Earth's surface. Still others like Vaccine Man and Homeless Emperor aim for utter extermination of the human race.

Mysterious Beings are especially concentrated in Z-City and Q-City. These monsters are the reason Agoni founded the Hero Association.

The Monster Association was created to oppose the Hero Association and cause chaos on Earth. The existence of this organization is proof that monsters, despite their innate compulsiveness to act rashly, can cooperate under an organized system. Mysterious Beings also seem to be able to detect the distinctive auras of other monsters as large numbers of monsters headed to Z-City to join the Monster Association.

Some Mysterious Beings, or parts of their body, can be edible for humans as seen when Saitama ripped off all of Kombu Infinity's leaves to cook his kombu stock and suffered no ill effects.

List of known Mysterious Beings by Disaster Level
Dragon=

Demon=

Tiger=

Wolf=

Unknown=

*There are also numerous unnamed monsters.

Types of Mysterious Beings
There are numerous types of monsters, and some monsters belong to multiple categories.

Natural Type: Sentient races that have been dormant and/or living in seclusion from the surface world.
 * Seafolk: A race of beings that originate from the sea.
 * Skyfolk: A race of beings that originate from the sky.
 * Subterranean People: The self-proclaimed "True People of the Earth". They live under the earth's crust, but grew too numerous in numbers and planned to take over the surface.
 * Terror Lizards: Dinosaurs that have come from mountains to take the surface of the earth for themselves.
 * Vampires: Ancient bloodsucking creatures with a long history.
 * Forest Tribe: A race of beings of unknown origin that have come to revive the lost greenery with fertilizer made of human corpses.

Unusual Type: The most bizarre type are the Mysterious Beings that were normal individuals whose change happened through performing an action obsessively.
 * Ugmons: Ugmons are grotesque creatures created from ugly humans.

Extraterrestrial Type (more commonly called aliens): Sentient beings alien from Earth. All members of the Dark Matter Thieves belong to this type and are the only known aliens.

Artificial Type: Normal beings or individuals which, by scientific means, gained superpowers or mutations.
 * Mutant: Mutants are creatures created through genetic modification.
 * Robot: Robots with artificial intelligence that pose a threat to humans are treated as Mysterious Beings. G4 is one such example.
 * Cyborg: Cyborgs are cybernetically enhanced living beings.

Humanoid Type: Mysterious Beings who are hard to distinguish from regular humans in terms of appearance. Do-S, Gale Wind, and Hellfire Flame are part of this type.

Non-Sapient Type: Some Mysterious Beings are just feral beings with no higher brain function, similar to that of a wild animal.

Human Type: A human who is designated as a monster without actually becoming one. Garou and Homeless Emperor are two such examples.

Infused Type: Mysterious Beings who are the result of a human or animal consuming the uncooked flesh of a monster. For example, a human who has eaten a Monster Cell or the crows who ate the Beast King's flesh. Several have displayed the ability to switch back and forth between their human and monster forms.

Disaster Levels
Mysterious Beings are assigned disaster levels to estimate their destructive potential. The levels are as follows (in order of increasing threat level): Wolf, Tiger, Demon, Dragon, and God. Although the disaster level system was created to estimate the threat levels of Mysterious Beings, it can also be applied to other threats, such as villains like Garou and natural disasters like the giant meteor. They give heroes and citizens an estimate of what level of danger to expect. The Monster Association also assigns disaster levels to its members.

Several factors are taken into consideration when determining a disaster level. These factors include aggressiveness, the difficulty of extermination, and combat prowess. Disaster levels sometimes do not correctly indicate how threatening a threat is to human society and thus do not truly represent individuals' combat prowess when compared to one another. For example, Gouketsu was much stronger than Bakuzan, even though both were classified as Dragon-level threats.

Disaster levels can be incorrectly assigned. These may be due to the Mysterious Beings hiding their strength such as Withered Sprout, ignoring context-dependent factors such as suitability to the particular event or an individual defeating monsters without reporting their true level of strength. While heroes engage monsters, they sometimes must update the Hero Association on the correct threat level. While some heroes stick to attacking weaker Mysterious Beings to maintain their ranks, defeating a high disaster level monster is very influential on hero rankings. Many heroes challenge these powerfully ranked monsters to move rapidly up the hero ranks such as Stinger who rose to A-Class within six months.

Trivia

 * During a Q&A session, ONE stated that Boros is an above-Dragon threat. This could mean that Boros is in an unknown level between Dragon and God or indeed a God-level threat, but it was never specified.
 * Orochi was similarly described by Murata.
 * Wolf is never used or even considered a disaster level in the anime.
 * This is most likely due to the near lack of Wolf monsters in the manga at the time.
 * God level threat is represented by a Fenghuang, which is rather ironic, as the Fenghuang is often associated with virtue and grace and has positive connotations.