Hydra is a natural satellite of Pluto, with a diameter of approximately 51 km across its longest dimension.  It is the second largest moon of Pluto, being slightly larger than Nix. Hydra was discovered along with Nix by the Pluto Companion Search Team in June 2005. It was named after the Hydra, the nine-headed underworld serpent in Greek mythology.[10] By distance, Hydra is the fifth and outermost moon of Pluto, orbiting beyond Pluto's fourth moon Kerberos.

Hydra

PDS Image Atlas

Cross-mission and cross-instrument searches for images and spectra from Cassini, Voyager 1 and 2, Galileo, New Horizons, and the Hubble Space Telescope