Once you have the instrument from the Tail Cave, it is time to head north. Go through Mabe village, and some kids will tell you that moblins attacked. Speak to Madam MeowMeow, and she will let you know that BowWow got kidnapped.
How To Rescue BowWow, Get Through The Goponga Swamp
Head into the Mysterious Forest and go back to where you found the Witch to make your Toadstool potion. When you see the Piece of Heart, use your new jump skill to get to it, then jump north to get access to a new area. Head east to locate the statue of an owl with a door beside it, and go to the entrance of the Moblin Cave.
Fight your way through the cave, then take out the Moblin King. He will throw spears and charge you. Let him charge, then dodge at the last second and he will slam into the wall, allowing you a window to do damage. Repeat this cycle until he dies. BowWow will be in the next room, inspect the rock to free him and bring him with you.
Once you have BowWow, you can head east, back to the Goponga Swamp, and he will eat the plants blocking your path between the hedges in the water. Make your way to the door, and he will devour the plants in front of you, and you can head inside.