Board Thread:Wiki-Related Discussion/@comment-1916997-20151007040019/@comment-11464223-20151010181745

Applegirl wrote: Species is for animals/beings that has in-show confirmation that there are or have been more than one of them in existence. Example: Fairies, dwarves, furies, dragons.

Creatures is for animals/beings that only one has been confirmed to exist within the show. However, it's also true some animals/creatures that are already in the Species category are also in the Creatures category. Example: The Shadow. Makes sense.