Originally Posted by 
Vyrerus
				
			 
			It was actually just created with the bosses, save Echidna, that appear in World of Darkness from FFIII. They upgraded Twinheaded Dragon to Five-Headed Dragon while making a Two Headed Dragon a trash monster (probably because we'd already fought a Hydra in the relic quest available at 50). They are Greek mythos references, but in FFXIV's case, it's more of wanting to reference the source Final Fantasy than anything else.
Edit: That's also the actual reason there are Xande Clones, Kunoichi, Shinobi, and all those other trash monsters as well, hehe. Though I think the Atomos segment is unique.