After getting out of the temple, the Owl tells you about the Full Moon Cello that you found and informs you that you must now head for the Goponga Swamp. Go back to the village. On your way, the children will tell you that BowWow, Madam MeowMeow's pet, was captured by Moblins! Go to her house (up, right) to learn more about it. If you have at least 200 rupees, go to the shop northeast of the village and buy the Deluxe Shovel (but you can do it later). Get out of the shop and if you still have some rupees, go back in to buy 10 bombs for 10 rupees (GameBoy versions only).
Get back into the Mysterious Forest. Go through the forest towards the Piece of Heart to the northeast and pick it up using Roc's Feather. Then, jump to the north to enter the Goponga Swamp.
Go right and get into the cave. Fight the enemies to meet the Moblin Leader in the third room. To defeat him, try to bring him to the middle of the screen, while avoiding his attacks. When he dashes towards the wall, he remains stunned for a few seconds, hit him swiftly with your sword and repeat this strategy four to five times. Hit the rock holding back BowWow to set it free and bring it back to its owner.
Madam MeowMeow will ask you to take it for a walk, so be it! Go back to the forest again, go to the chest that was withholding a Small Key, go on northwest and get out of the forest. Now go east to reach the Goponga Swamp. The sign says that this place is dangerous, except for BowWow. Go forward into the swamp, BowWow will take care of all the enemies along your way, and look for a cave to the northeast. This is the entrance of...