After you get the instrument in Tail Cave, you learn that BowWow has been kidnapped. Your search leads to Goponga Swamp.
Find the Moblin Hideout
Go north. The kids who were playing catch will run up to you and say that some Moblins went to BowWow's House. If you go there, you find that BowWow was kidnapped. Madam MeowMeow doesn't have any more information.
You have seen some Moblins in Mysterious Forest, and if you go to a phone building and talk to Ulrira, he says that the Moblins can be found in Mysterious Forest or Tal Tal Heights to the north.
Go to the Mysterious Forest, and go northeast until you reach the place with the Piece of Heart in the middle of the holes. Jump to the Piece of Heart, then jump north to a path that you couldn't reach before.
Go northeast and you will see a bird statue. Go into the cave next to that statue to find the Moblin hideout. Fight the Moblins and go through the cave. You will find King Moblin.
Avoid King Moblin's spears and allow him to come closer to you. After a moment, King Moblin will wave his arms, then charge toward you. Step out of the way so that King Moblin crashes into the wall. Hit King Moblin while he is stunned. Repeat the process until he dies.
Find the Next Dungeon
Afterward, go east to find BowWow. Touch the rock that he is attached to. You will carry him with you. Leave the cave and go west, then south. Jump over the holes in the ground and go west. Continue going west through the forest until you can go north. Then in the screen with a house across a gap, go east until you reach the swamp, then go north.
Stand near the Goponga Flowers in the path and wait for BowWow to eat them. Then go east until you reach a cave entrance that is blocked by more flowers. Wait for BowWow to eat them, then go into the cave. This is the second dungeon: Bottle Grotto.