What Greek oder Roman Myths has Rick NOT mentioned?

Rick Riordan has based all the Percy Jackson and Heroes of Olympus Bücher based on Greek and Roman Myths, so it's hard to know which Myths he's mentioned and which ones he hasn't.

But if we find out some Myths he hasn't added yet we might be able to unlock some clues as to what will be in the son of neptune!!!

if Du know any myths Rick has NOT mentioned in both the Percy Jackson series and the Heroes of Olympus series TELL ME!!!