The big difference between fantasy and mythology is ecology. In a fantasy world, monsters have an ecological role in the world; dragons breed and make more dragons that grow and spread. In a mythological world, monsters are more unique: there might only be one dragon, or if there are multiple dragons each might have unique powers and lore.

There’s a benefit to both approaches. Things get silly in D&D when every monster has families and comes from a long line of monsters, when writers need to stop and think how gibbering mouthers or hydras reproduce. But if every monster is a unique flower that greatly diminishes the ability of heroes to fight Name monsters.