The three broadest monster varieties are "natural", "fantastic", and "otherworldly". Natural creatures are your basic plants and animals; these don't have to be "real", but they need to seem like they could be real. A horse with a poisonous bite, for example. The natural varieties are Plant and Beast, with some sub-varieties for particular shapes or bundles of features: Mold, Tree, Herd Beast, Bird, Fish, Bug, Humanoid.
Fantastic creatures are unnatural variants of natural creatures. They will have a keyword that is one of the natural varieties listed above, but with one or more of these prefixes:
- Monstrous: Significant alteration of the base form, such as enormous size, extra/missing limbs or organs, or exaggerated physical abilities or defenses.
- Hybrid: A mix of body parts of two or more natural creatures. You can also include inorganic materials in this category, but you would normally replace the keyword "Hybrid" with "Metallic", "Stone", or other broad material type.
- Enchanted: Magical powers. Some unnatural effects that are part of a venom, stench, breath weapon, or secretion might be considered Monstrous, but many others, and all gaze attacks, auras, immunities and outright spells should be considered Enchanted.
- Cursed: Involuntary, spiteful magical power, usually triggered by killing the creature, surviving its attack, or failing to perform some act of appeasement. Cursed creatures are usually themselves being punished, perhaps as a result of killing or surviving the attack of another similar creature (for example, the werewolf.)
Or, they may be one of these additional varieties, with or without one of the above prefixed: Ooze, Undead, or Automaton. Oozes are found naturally, of course, but the ooze as a monster is significantly different from the mostly harmless slime mold or the microscopic amoeba. Undead are either skeletal or corporeal, and Automatons can be made of a wide variety of materials. All three varieties are inherently fantastic even without an additional modifier. Shapeshifter is also an inherently fantastic variety and are always considered either Enchanted or Cursed; if used by itself, one of a Shapeshifter's forms is assumed to be Humanoid; otherwise, it should be combined with another keyword, like Shapeshifter Bug.
Otherworldly creatures may resemble a natural creature or hybrid (prefix a natural or fantastic variety with "Unearthly",) or they may be shapeless, have no fixed form, or a very unusual form (in which case, they can be labeled "Unearthly Being".) They are always considered Enchanted, even if they have no noticeable power, generally have some special purpose, and are frequently unique. Some specific varieties exist: Fiend, Angel, Godling, Spirit. Each variety typically has a package of common abilities, like becoming ethereal; there may be several such packages, such as "demon", "devil", or "hag".
Enchanted, Cursed, and Unearthly creatures are all held at bay by a magic circle. Unearthly creatures with Chaotic alignment can usually be turned away by clerics, although perhaps only by a powerful cleric.