[Tiktaalik]

Nintendo is an iterative developer. They prototype everything and they don't start on any final art or narrative development until the gameplay fundamentals are solid.

For example the Iwata Asks interview with the Splatoon team mentions that the early prototypes for the title were just experiments where boxes were running around in a box world shooting colours at one another. Once that was fun and the controls felt good they figured out everything else about what the game was going to be.