Because many people’s experience in fantasy writing is limited only to the Harry Potter series (and perhaps the Lord of the Rings trilogy…no, Twilight is not fantasy, no matter what any rabid glittery fangirl insists), there is often a misconception that many of the creatures that appear in the books were created by J. K. Rowling.
Great author though she is/was, she did not come up with several of the fantastical creatures, something which she has attested to herself. Neatorama has compiled a list (and included the back stories) of the creatures that have much older origins than a late 20th century teenage fiction series. Though the sources might all be from Wikipedia (which, yes, is slammed by every single university professor ever), they do corroborate with several of the folklore books I read when I was younger and, anyway, it’s all mythological yeah? The point is that they existed before Harry Potter. Check the article out here.