I notice that it's not actually fixed, though it may be helped, by digging deeper into one folklore. Throw in a nuckelavee instead of a minotaur to menace your elves. Alter the werewolves plaguing the Greek gods to match the Greek legends of them. But even those sources are not marvels of consistency. Do trolls turn to stone by daylight, or not? And if there's variation, how does it work? Are fairies ghosts or not?
There are in fact two main tools that help in my experience: lumping and splitting.
Lumping covers a wide range of things. At the highest level, it means a unified metaphysical sytem, such that you can explain when the dead who do not sleep peacefully in their graves are sometimes vampires and sometimes (like the Cauld Lad of Hylton) are nastier sort of brownies.
At the most direct, it's the characters' point of view. Just as Spaniards wrote back to Spain that there are tigers in the Americas, only spotted instead of stripped, just as Tacitus wrote that the chief god of the Germans (Odin) was Mercury, so to would an Indian write back that in Scandinavian parts, the rakshasa sometimes turn to stone in sunlight, but they are more likely to be friendly, even if he adds that the local term for them is "troll". Or a Pole write back that the dragons in Germany are enormous and often can't change shape -- that's one where the widespread use of the term stuck but good.
And splitting arises when you realize that many creatures and things in the folklore are unique. It is the folklorists, not the folk, who label Penny Greentooth as a nursery bogie, and Cauld Lad of Hylton as a ghost and brownie. The characters don't have to regard them as a species. This can help a lot where you don't have carry the baggage along with the name and so have a freer hand.