Does Harry Potter have unicorns?

Yes!

Does Harry Potter have giants?

Yes! Hagrid, Grawp, and many others.

Does Harry Potter have Godzilla?

No!

Does Harry Potter have mermaids?

Yes! The merfolk.

Does Harry Potter have dragons?

Yes! In the Triwizard tournament, at Gringotts Bank, etc.

Does Harry Potter have zombies?

Maybe? The infieri?

Does Harry Potter have Satan?

Maybe? Voldemort?

Maybe? Harry Potter?

Does Harry Potter have ghosts?

Yes! Nearly-Headless Nick, the Grey Lady, and many others

Does Harry Potter have vampires?

Very little! A few brief mentions.

Does Harry Potter have elves?

Yes! Dobby, Kreacher, Winky, and many others

Does Harry Potter have Merlin and King Arthur?

A little! Merlin is mentioned on occasion.

Does Harry Potter have Frankenstein?

No!

Does Harry Potter have werewolves?

Yes! Fenrir Greyback, Remus Lupin, etc.

Why doesn’t J.K Rowling invent her own creatures?

Why does she change them (often, a LOT)?

Why does she exclude (or give little attention to) some?