Styx and Kerberos, the new names for the two Moons (formerly P4 and P5), continue the ‘underworld’ theme associated with Pluto’s name (Pluto being the Roman God of the underworld named after Hades, the Greek god of the underworld, death and the dead).
The new names were chosen by the IAU (International Astronomers Union) from the Pluto Rocks Internet poll vote. The IAU is responsible for naming celestial objects.
In mythology, Styx is the river over which the souls of the dead are ferried to the underworld; and Cerberos the multi-headed dog which guards the underworld. Charon is the ferry-man who carries the souls across the River Styx. Nix is Charon’s mother, and is the Greek goddess of darkness and night; Hydra is the nine-headed serpent who battled Hercules in Greek legends. The number nine is reference to Pluto’s status as the ninth planet from the Sun.
For more about the new names, see: