What is a "monster"?

The word "monster" (with or without a capital M) has any number of definitions in the context of fantasy roleplaying games:

• generic term for anything the party fights
• specifically magical or unnatural creatures (i.e. any animals that don't exist on Earth)
• unique solitary existences, like the monsters of Greek myth
• weird non-sexual genesis, such as the basilisk and cockatrice being hatched from a cock's egg hatched by a frog or snake... or divine fiat, tutelary deities, magic weirdness, chaos mutations, spontaneous generation, etc
• anything which eats people or otherwise exists solely to wreak havoc, which may be applied to humanoids and beasts alike

The Earth-centered division is the laziest and weirdest criterion. If you take this at face value, this implies that the gods--who created the fantasy world and provide clerics with their spells--knew about our Earth and arbitrary used it as a yardstick to determine which is and isn't magical or unnatural.

This continues into 5e: the "monstrosity" type is a catchall category with an arbitrary and inconsistent definition, which encourages lazy design. Supposedly it is limited to magical and unnatural creatures (although the beast type includes magical and intelligent beasts), but some monstrosities are non-magical and/or natural.  It includes fantastical creatures from medieval bestiaries, monsters from Greek mythology, and monsters invented for D&D.

The path of least resistance is to rationalize the type with in-character logic. Since I'm using the winds of magic, I rationalize the monstrosity type as being governed by the orange wind of disorder and entropy. This doesn't make them unnatural, as the nature/artifice division is governed by another pair of winds.

Aside from that, how does being a monstrosity make them different from beasts or humanoids? It would be simple to rationalize that they are always hostile, pointlessly destructive and difficult to train if the option exists, if things like lawful good centaurs didn't get in the way. Alternatively, I could rewrite the existing backstory to fit my rationale. If monstrosities are governed by the orange wind, then that means they fall into the weird non-sexual genesis criterion. All of them, not just some of them like nagas or medusas.

I would prefer to simply remove the monstrosity type, but that leads to balance issues because we don't want druids wildshaping into bullettes or wizards charming medusas. In that case, the obvious solution is to bake the desired limitations into the effects (as Pathfinder and 13th Age do) or a level adjustment mechanic rather than rely on types. Druids may already wildshape into crag cats, which have spell turning and nondetection, so types already fail at maintaining game balance. To add insult to injury, the designers know this and don't care.

When I discuss specific monstrosities in later posts, I will be sure to give them alternate types for use with any no-monstrosities optional rule.