SCYLLA AND CHARYBDIS

To nymphs from Greek mythology who became monsters. As a punishment for causing floods, Zeus turned her into either a whirlpool or a monster that caused whirlpools (accounts vary). FF3's Charybdis appears to be some kind of coral.

Scylla was loved by the sea god Glaucus, whom the jealous sorceress Circe wanted for herself. So, Circe tricked Scylla into drinking a potion that turned her into a monster.
Descriptions of the transformed Scylla vary. Sometimes she's shown with a lower body made up of dogs and serpents, other times with multiple, snake-like heads.
The FF3 Scylla appears to be a combination of the two, with multiple dog-like heads and legs and a humanoid torso. As there was only oe Scylla (well, one monster called Scylla) in Greek mythology, there were no Scylla monarchs - thusly, the Queen Scylla is Square's creation.

Scylla and Charybdis occupied the "dire straits" - sailors such as Oddysseus who wanted to pass through the straits had to choose which monster to face.