One of the most important items in Baldo the Guardian Owls is the Guardian's Horn. To get this item, you must travel to the Owl Tower which lies to the north of Kidoge Village.
Once you enter the tower, you will fall through a hole in the floor. Once you are on the lower floor, you will have to complete a puzzle to proceed and earn the horn.
You will notice two gold squares on the floor on opposite sides of the platform. Push the stone pillars until they cover the gold squares. The spots are marked by a red circle. These will open up two eyes which are marked by the blue circles.
When you are close to these eyes, they will change from orange to blue. You will have to have Luna, your companion, stand in front of one of the eyes which are also marked by a red circle. You will have to stand on the other. Both eyes will turn blue and you will have completed the puzzle.
Doing so will cause the Guardian's Horn to fall from the ceiling. Enore, an owl, will also fly down. The owl will tell you about the Guardian's Horn and instruct you how to play the Owl's Song. The Guardian's Horn can open doors, portals, and other devices. You learn other songs from the Minstrel later on. Using the horn requires a Magic Orb.
For more information on Baldo the Guardian Owls, check out How to complete the Cato Cabbage Quest in Baldo the Guardian Owls on Pro Game Guides.