Mythology of Hydra
Hydra guards the cup of water from Corvus, the Crow, forever denying him, a drink of water.
Hydra was a nine-headed monster in the form of a long, writhing water snake. It attacked, killed and ate anyone who ventured near its swampy abode in Lernea. To slay this dreadful beast was the second of Hercules' twelve labors. This was an enormously difficult task, since every time Hercules succeeded in cutting off one of Hydra's heads, two would grow where the one had been. Added to this, Hera, who had always been resentful of Hercules, sent a giant crab to nip and bite at his feet as he attempted to slay the monster. He finally achieved this after instructing his nephew, Iolaus, to burn each neck stump immediately after he, himself, had severed the head, thus preventing any re-growth.
Hydra is a very weird constellation. It is the largest constellation on the sky (with Virgo close behind), but is strung out from Libra to Canis Minor along the southern horizon over about 90 degrees. Believe it or not, it used to be even bigger, but like the old Argo Navis constellation, it was split into four parts. Various stellar cartographers, including Flamsteed and Hevelius, broke the old Hydra into Sextans (the sextant), Crater (the cup), Corvus (the crow) and a new, reduced Hydra.