you find it in the giant clam in the water where it says dive. :)
under they sea at Triton's palace
The Hydra is in his underwater lair. Swim out from the beach to the underwater maze, then go right, down, and left to find the Giant Pearl and the entrance to the Hydra cave.
In the realm of Poseidon, there is a shallow sea with a beach. An area of darker water marks the location of the underwater maze that leads to the Hydra cave. Just outside is the Giant Clam that has the Giant Pearl, another of the 5 sacred items.
You have to go underwater which is left to Aphrodite and you have to find a clam. It opens and closes. Try to get the pearl (which is inside the clam) when the clam is open.
Just stay along the top, along the right side, then along the bottom until you reach the Hydra cave. The Giant Pearl clam is just above the entrance.
go to posidens lair and go till u find the clam. be sure to collect air bubbles
On the island
Orion.
Mythology island, on a statue in the museum
Find him and click on him.
To give Zeus the five sacred objects on Mythology Island in Poptropica, you need to find and retrieve each one from different locations on the island. The sacred objects include Poseidon's conch shell, the golden vase, the giant pearl, the flower from the Garden of the Sphinx, and the shield from the Minotaur. Once you have all five items, return to Zeus at the top of Mount Olympus to complete the quest.
The item from the Hydra is a scale, not a hair. The item from Cerberus is a whisker. Find the Hydra cave in the underwater maze in Poseidon's realm. (The giant pearl is right outside.). Knock out the Hydra's 5 heads one at a time by jumping up when he tries to hit you, and landing on the head. Get a scale when he is knocked out fully.