Monster

Monster (魔物) is a term that refers to all Magicule-based lifeforms. As magical beings, monsters do not require the essentialities of normal lifeforms—such as eating, drinking, sleeping, or breathing—and instead only require sufficient magicules for sustenance.

Monster Origins
There are three manners in which a monster may come into existence.
 * The first is called spawning, where the monster is born out of the magicules either latent within the surroundings or let off by a "parent" monster.
 * The second is straightfoward and has either a singular monster or a pair of compatible monsters engage in regular biological reproduction.
 * And the third involves the mutation of non-magic lifeforms—plants, animals, humans, demi-humans, spirits, or magical beasts—into magic lifeforms. It occurs when the subject is exposed to more magicules than it can handle. While this would normally result in death, the subject lifeform can occasionally be reborn as a monster.

Majin
The term "Majin" refers to monsters that possess proper intelligence and rational thought similar to humans and demi-humans. A monster can be a majin from birth (such as Vampires, Giants, and Demons), become one later in life via evolution, or be born by the monsterization of an already intelligent lifeform, i.e Humans and Demi-Humans.

Unique Monsters
On occasion, individuals of a particular race of monster may be born with certain characteristics, traits, or abilities not found within the typical member of their species. These individuals are called "Unique Monsters" and are usually much stronger and smarter than their ordinary counterparts. Examples include the Orc Lords and Rimuru Tempest.

Trivia

 * While many Demi-Human races, such as Goblins, Orcs, Lizardmen, and etc are often called "Monsters" the term used for them is 魔族 (Mazoku) which collectively refers to the enemies of humanity regardless of their status as magical beings.
 * As there is no set standard for members of their race and because of the circumstances behind their birth, True Dragons are each individually considered "Unique Monsters".