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”?