It’s an anime movie (1970s – 1980s) about a young mouse who receives a letter that his mother has given birth to a baby brother. He decides to build a ship with his friends, a penguin and a cat, and sails off to the island where his mother lives. A storm hits one night, and the friends become stranded on an island filled with pirates. One pirate, a monkey, recruits the cat in a rock-catapulting war against a bear, who recruits the mouse. They all work together, however, to uncover a treasure in a cave. When the cave begins to collapse, a parakeet shows them the way out, and the three friends reach their original destination riding on the back of a purple whale who eats apples.

I remember the opening scene features the three friends sliding down something and screaming.