Someone asked me recently where I got my sources for folklore creatures, since some I know about are relatively obscure.

For anyone interested, or people looking for some cool ideas to populate their own Fantasy world with, my “Holy Trinity” on folklore and superstition sources are

The Medieval Bestiary

Mythical Creatures List

And most important of all

Internet Sacred Texts Archive

Of course, you always need to do a bit more of in depth research of specific creatures, but these sources cover a large enough range of subjects. And as always, Wikipedia will always provide a decent overview on specific creatures and myths for you to get the basics