Momotarou is one of the most common Japanese fairytale. The protagonist of this story is Momotarou who literally is born from peach (Momo). The story tells the importance of making friends and having the courage.
Once upon a time, an elderly couple who don't have their own child picked up big peach flowing through a river. They split it by kitchen knife and find the baby in it. The baby is named Momotarou after the peach and looked after by the couple. He becomes adult and make his mind to slay the demons that live in the island far from his hometown. On the way to the mission, a monkey, a dog, and a pheasant are joined the party by feeding the dumblings (Kibidango), which is actually a local sweets at Okayama prefecture. As a result of the battle, they conquer the demons and bring their treasures to his hometown.
I have no idea why they have to fight with the demons and rob them of their belongings because what demons do is unclear in the story.