Many fantasy stories contain creatures of the very odd to the strangely familiar. Writers seeking to do something different from the norm or even put a new spin on the usual will find this compendium to be useful.
The Element Encyclopedia of Magical Creatures: the ultimate a-z of fantastic beings from myth and magic by John and Caitlin Matthews is the place to begin your creature research. Discover creatures from South American traditions, Chinese folklore, Greek/Roman myths, and more. Some entries, like Boroka, are brief paragraphs, while others, such as Baba Yaga, are more detailed.
So far, I've only flipped around, stopping to read what catches my eye. Sometimes I find an unexpected origin to a creature or character in one of the books I've read. Such as right now. I have the page open to where Banshee/Beansidhe is listed. "The very first being to set up a keening cry was the Irish Goddess Brighid, one of the Tuatha de Danaan; she wailed for the death of her only son, Ruadan, and that was the first keening ever heard in Ireland."
In The Wandering Fire, book two of the Fionavar Tapestry, Guy Gavril Kay utilized a variant of this into his culture. Every Midsummer's Eve, the priestesses perform a ceremony honoring the death of the Beloved Son, Liadon. Dana's power holds sway on this sacred night, and this plays an important role in the events of the second half of the story.
Nor was that the only myth utilized in the Fionavar Tapestry. He also wrote about beings called the lios alfar and svart alfar. These are based on the two tribes of Alfa, or elves. Many of the Celtic gods and goddesses play nearly identical roles in Fionavar, some with slight twists to their names. Cernunnos/Herne is Cernan of the Beasts. Nemain is one of the goddesses of war.
Now there are many mythical figures left out. Apollo and Artemis have no entries of their own other than brief mentions in the entry for Cerynean Hind and Lords and Ladies of the Animals. I haven't been able to find a mention of Dana, the Great Mother, in some form at all. But over all, you will find many deities, larger than life characters, and creatures of all kinds you will likely be discovering for the first time.
If you are looking for a source of inspiration or just a bit of something to spice up your world, whether it be fantasy or science fiction, start with this book.