One common theory among all Pokemon Fans, Beginning all the way back in the First Generation, is that Cubone is actually the baby Kangaskhan holds in its pouch, one that wears its mother's skull. It's a tad disturbing theory, but this game actually lends to the credence of such. You see, when you discover a Cubone in Battle, there is a small chance, but still a chance indeed, of Kangaskhan coming to its aid.