Hiruko

Hiruko(蛭蟲) is one of the Six Ancestors, of the Scylla bloodline. Although her appearance is just like a normal girl, it is said that she's hiding numerous tentacles and abnormal organs in her body. It is also said that she has much more superior power than modern monsters, but the specific info is unknown.

Monster Girl Quest trilogy
Hiruko is mentioned by Seraph Eden in Chapter 3.

When Eden transforms into her boss form, she mentions that she hasn't used that form since she fought with Hiruko in the Great Monster Wars a thousand years ago.