Mythical Creatures

Yes, they do exist, just not on this physical plane.
There are spells that open a gate, summoning the mythical creatures, and there are also spells that release your inner ____
, which lets you become a creature. But these spells don't work for everything.

For the above post, you can not become a mythical creature, believing is a powerful tool, in which can consume you if you believe you are a creature; example: You believe you are a werewolf for a long period of time, your body will start believing in it (you will not transform), in which will cause your mind to believe in it, you will start feeling like you are becoming this mythical creature, but in all actuallity you are just imaginating it.

Mythical to an extent. There's a hint of truth to every myth. Let's address each one individually.

For werewolves, there are individuals who have actually deluded themselves to believe they are lycanthropes. It's a mental disease.

The closest thing you'll find to dragons in history are dinosaurs.

Mermaids are what men at sea imagined when looking at a sea creature after long periods of time away from their home and wives.

There have been and still are many vampire cults. Individuals have been known to have sever skin disease which causes their skin to burn at exposure to strong light. Though the closest encounter you'll have to an actual vampire is a mosquito.

As for these mythical creatures existing in all their glorified forms in other "astral" worlds, that depends on your individual beliefs.