I always assumed the difference between Beasts and Monstrosities was very simple: beasts are natural creatures that exist in our world, monstrosities are unnatural creatures that do not fit into any of the other categories. However, I was thinking about it and I realized some things that are classified ‘Beasts’ would be monstrosities by that definition, such as giant fire beetles.
The Monster Manual defines beasts as
non-humanoid creatures that are a natural part of the fantasy ecology.
and monstrosities as
monsters in the strictest sense- frightening creatures that are not ordinary, not natural, and almost never benign.
Is the distinction, then, simply one of origin? A fire beetle is a beast because it evolved naturally, while a Roc is a monstrosity because it was created directly by a Giant? This argument does not hold if you look at the roper, whose flavour text states
The roper is an evolved, mature form of piercer.
Is it one of alignment? A roper is a monstrosity because it is neutral evil, and a giant fire beetle is a beast because it is unaligned? But a roc is a monstrosity, and is also unaligned.
What, then, makes a creature such as a roper or roc a “monster in the strictest sense”, and a giant fire beetle or a stirge a “natural part of the fantasy ecology”?
Best Answer
Your question is looking for a definition beyond the definitions provided in the rules. There isn't one. Creatures are the types they are because that's how they've been defined in the rules.
The Rules
The terms "Beast" and "Monstrosity" are individual examples of "creature type", which is explained in the Basic Rules .
For beasts it says:
For monstrosities it says:
But what's the difference between beasts and monstrosities?
That's it, what the definitions say, that's the difference.
There is no further explanation for what makes some things beasts and some things monstrosities. Furthermore, the rules do not provide further detail of what "monster in the strictest sense" and "a natural part of the fantasy ecology" mean, beyond what those phrases mean in everyday English. You can certainly look at the categories and at examples and look for patterns, but in the end, the rules say what the rules say.
The usefulness of creature type
It's pretty clear the usefulness of creature type. We can then have rules that say "druids can shapeshift into beasts", and the spell planar binding can "bind a celestial, an elemental, a fey, or a fiend". The basic rules say "certain spells, magic items, class features, and other effects in the game interact in special ways with creatures of a particular type." But that's not what you asked. You want to know why some creatures are thrown into one bin, and why some are thrown in another.
The binning problem
The rules categorize some creatures one way, some another.
Examples of beasts are various dinosaurs, brown bear, deep rothe, spider king.
Examples of monstrosities are aeorian absorber, assassin bug, death dog, hippogriff, winter wolf.
Generally, what are listed as beasts seem close to beasts in the real world, and what is listed as monsters seem, well, monstrous. You can look at the descriptions of the creatures, and look at the definitions, and either agree or disagree with the designers' decisions. While most people might agree on many examples, there could well be cases where a creature could reasonably be classified more than one way. But those would be subjective judgement calls.
In the end, we have to conclude that at some point content designers examined the creatures and assigned a type, but unless definitive statements can be found, the "why" remains a question of unanswerable designer intent. Metaphorically, we can imagine that, having come up with the definitions above, they examined each creature as it was being added to the 5e rules, and said, hmm, this looks more like a beast, this is obviously a monstrosity. Maybe they argued about it. There's nothing in the rules that defines the definitions beyond the definitions. Without statements outside of the rules, we don't know.
The challenge of categorization is not unique to D&D. There's even a whole Wikipedia article on it. It's useful to divide things into categories, but categorization itself is a simplification. In the end, you define bins, then you throw things into the bins, then you argue about it. Look at taxonomy. Taxonomists argue endlessly about what goes into which bin.
Making your own monster
The DMG provides guidance for creating your own monster. In Chapter 9: Dungeon Master's Workshop, Creating a Monster, Step 3. Type, it says:
In other words, look at the bins, look at the monster, throw it in one of the bins.
History of creature type
Finally, the article Creature type in the D&D Lore wiki provides a history of the concept of creature type, which is interesting, but does not provide much elucidation on what makes a creature one type and not another.