The Hydra is located in its cave, which is an underwater air pocket in the realm of Poseidon. To get there, you must swim through an underwater cavern. When battling the Hydra, each one of its heads will rear back and plunge down towards you. When that happens, jump and land on them. This will knock them out. There are five heads and you must make sure all of them are knocked out. After that, click on it to retrieve its scale.
A Poptropican after stomping on one of Hydra's heads.
Hydra after being defeated.
One of the Hydra's scales, which is one of the Sacred Items.