Hydra is the three headed boss for the swamp snakes. He is two blocks tall and two blocks long. In the iOS version of Block Story, Hydra, along with Swamp Snakes and Dark Werewolves, only spawns at night. This allows for safe collection of its spawner during the daytime.
The hydra spawns in swamps near jungles or in the Sky-lands, there may be sand and the water may be uneven. It appears rarely and infrequently but its spawner is easy to craft once you have killed it, requiring only klorium and a swamps snake spawner so it is best to just kill it and make your own spawner once you've seen it.
He is green and scaly with three heads.