Most of March’s monsters have been monstrous of face, but there are other ways to find yourself outside the natural order of things.
Sirens are curious creatures. They sing sailors to their deaths, yet they are not creatures of the sea- no more than the sailors are, anyway. They are, however, associated with sea creatures. And birds. And flowers. They are beautiful and tempting and the reason the Barnes and Noble Leatherbound Edition of the Iliad and the Odyssey features bondage on the cover.
The sirens were the children of a river god and muse, or a human, or the earth itself; it isn’t terribly fixed, mostly because for the stories of the sirens, their origins don’t matter. The Minotaur’s origins were a part of its monstrosity; the sirens are monstrous because of their effects on sailors, and perhaps how they appear (depending on how they are being portrayed), but their pedigree is one of the least drama-ridden in Greek myth.
Stories of the Sirens aren’t about families or betrayals or rape or dynastic power; their stories are about adventurers. They exist almost exclusively as obstacles. They are the NPCs of ancient Greek myth. And this lack of material about who they are and where they came from means there is all kinds of room to invent it, even in mostly historical/mythological games.
Do the sirens understand the effects of their music? Do they understand humans as people, or does the only occasionally musical speech of humans seem as simple and nonsensical as the baying of hounds? Are they lonely on their island? Do they sense time passing? Can they remember? What happens when the singing stops?