Cerberus

Cerberus is a three-headed canine in Greek Mythology. It is seen guarding the Underworld on Mythology Island.

Role on Mythology Island
If you go to Apollo's temple on the roof, you'll find a hint: As you look at a glowing olive, Athena says, "Music soothes the savage beast." Inside the temple, go to right so you can get a reed pipe. Go to Euterpe, the muse with the reed pipe, and do what she says. She'll then give you a memory test. If you pass, she'll give you the musical score and tells you that you can use the music to put Ceberus to sleep. You'll find him at the other side of River Styx. Zeus asks you to get one of his whiskers, which you soon discover that it is impossible, since he bites you if you try.To defeat him, use the pipe and a song to put him to sleep, allowing you to take one of his whiskers.

Role In Poptropica Adventures for Nintendo DS
In the events of Poptropica Adventures for Nintendo DS, Cerberus got very ill and Hades was trying to create a cure. Since Hermes had lost his winged hat and is unable to deliver anything, he asked you to deliver the final ingredient to Hades. Once it gets there, you must help him by catching the different liquids as he throws them down to you, then he will jump down and create a medicine to cure Cerberus.

Trivia

 * He's about three poptropicans high.
 * He also has three tails.
 * It is similar to the Hydra in a few ways. Both are found in Mythology Island, both have more than 1 head, and both have to be put to sleep in order to obtain an item from their bodies.
 * It wears golden earrings (one for each head) and three spiked dog collars.