The Hydra is a strong boss that was added in version 2.50. He can be summoned at the Ominous Altar (found in water above the Mammoth's den) by unloading four raw meat around it and placing a Blue Herb (or any other special herb, which special herb determines the Hydra's spawning location), Mushroom, River Root and Elk Hide inside the Altar in that order. The Hydra appears between the North West and South West island at the end of the map (the left big green circle) and it wanders the same paths as the trading ships. If you kill the Hydra he spawns two lesser hydras. If you kill a lesser hydra two more lesser hydras spawn. The only way to kill the Hydra is to kill all lesser hydras at roughly the same time. If the lesser hydras are not killed they will reform into the main Hydra after a time.
Has a poison attack like a short version of snake poison.
The Hydra drops two hydra scales for every lesser hydra alive at the end. For example, killing one of the two lesser hydras that immediately spawn from the hydra corpse gives nothing, but if the three lesser hydras which would be alive were killed all at once you would get six scales.