Anybody know? :)

You need to collect all nine pieces. Afterwards, go to Games>Treasure Maps>Unknown Island

Once you complete the map, you'll be taken to the island. You'll then meet Merana. After doing her quest, she'll turn your pet into a Hydrus. c:

Click here to view all the species that can be coloured Hydrus.

Thanks a bunch! :)

also at that Island you can use the Oceans Depths vending machine. It gives you wearables and anything that has blue or ocean.

also that vending machine gives you Riptide, which is a legendary weapon :) and two minions.