Hydrus: Collect all 9 of the Uncharted Land Map pieces, have all the pieces in your inventory and go to Treasure Maps (in Games) to turn them in. After a few minutes, you will be given access to the Unknown Island where you will need to complete a quest for the sea-witch named Merana. When you have completed the quest you will be allowed to change one of your pets to Hydrus, so long as the pet is currently available in that color.