In Harvest Moon: Tree of Tranquility it is unlocked after you've finished Daren's Rainbow. Talk to Pascal at the dock, who will ask you "do you want to go to Toucan Island?". Say yes, if you want to go to Toucan Island, and you will travel there via ferry. The first time you visit Toucan Island, Sue will ask you to deliver a message from her to Selena. If you deliver this message and return to Sue on Toucan Island she will reward you with a Manta Ray.