Get the grappling hook at the far side of the attic. Go back downstairs and you will see the Jersey Devil through the window. Go outside and you'll see it fly up. Use the grappling hook to climb up the tree. Get the eggshell from the nest at the treetop. Go back to Mews' mansion and give Mr. Mews the eggshell.
a raccoon
You don't get immortality, even on Mythology Island. Collect evidence at the four locations (and later a fifth) to prove the existence of the 4 cryptids on Cryptids Island. (see the related solution) (see the video links below)
You get the Island Medallion after you prove the existence of three of the four cryptids, and rescue Bigfoot from the evil Gretchen Grimlock. (see related question)
You don't have to get the giant squid on Cryptids island because if you watch the video you'll see it's already confirmed.
When you see the raccoon in the attic, keep going right and you will find a grappling hook and rope. When you come down, the Jersey Devil will fly away and you can climb up the tree with the rope.
He is not on Cryptids Island. He is on Counterfeit Island.
The name of the Poptropica island is "Cryptids Island." (see related question)
There are animals on many Poptropica islands. There is a Safari on Nabooti Island, cryptids on Cryptids Island, and in several quests. See the related questions.
There is no Nessy Island. The name of the quest is Cryptids Island. (see related question)
There was no way to see the actual monster on the Cryptids Island preview. Likewise, there is no way to see Nessie with the telescope in the actual quest. All you see is items disappearing into the water. (see related question)
there are no tickets on cryptids island try counterfit island
You cannot buy or get chocolate on Cryptids Island.