Despite its size, Hydra contains only one reasonably bright star (Alphard, α Hya). It also contains the radio source Hydra A.
In Greek mythology, the Hydra is the many-headed serpent which Heracles defeated. It had the unusual property that whenever one of its heads was severed, two new heads immediately sprouted from the stump; Heracles overcame this by immediately cauterizing the stump with a torch.
The term "Hydra" is often used today when referring to a problem that seems to share this sort of characteristic, for example when attempts to suppress a particular piece of information result in it being disseminated even more widely.
Hydra is also a Greek island.
Hydra is also the name of a distributed operating system developed at Carnegie Mellon University.