Stheno

Stheno is a gorgon sister of Medusa. She appears in The Son of Neptune.

The Son of Neptune
Percy first saw her on a Bargain Mart store posing as a greeter. At first Percy thought that her name is Beano until he decoded a big green button that said "Welcome! My name is Stheno". She is described by Percy as appearing as a dumpy old grandmother but with rooster feet and bronze boar tusk sticking out of the corners of her mouth. Percy already killed her but she, along with her sister Euryale, just kept reforming possibly due to being in contact with the earth, Gaea's domain.