In astronomy, Hydra is the name of a constellation, lying near the celestial equator. It begins south of the constellation of Cancer and extends eastward, almost all the way to the constellation of Libra.
The inhabitants of the Hydra system supposedly are one of the races visiting Earth. According to JD Stone, who claims to be quoting PoL, they are skilled in agriculture, archeology, and creating with their hands from the substances of the Earth.
(PoL = "The Prism of Lyra" by Keith Priest and Lyssa Royal).