It took numerous twists and turns for Splatoon to become the hit game it is today. Before the Inklings were ever conceived, Nintendo considered using existing IP like Yoshi in the game, and at an even earlier point in development the game starred blocks of tofu as its protagonists. Some of Splatoon's developers recently discussed these early prototypes at GDC 2018, and they even revealed footage of the tofu build.
As you can see, the earliest build of the game was much simpler, but the basic concept remains today. Players controlled black and white tofu-like blocks and shot ink to claim territory and to attack opponents. Of course the final build ended up being quite a lot more colorful.
Nintendo also unveiled beta footage of their new Switch IP, ARMS, at GDC. Like the early Splatoon footage, this build of ARMS is much simpler than the commercial product, but much of the core gameplay is in tact. The original prototype also included mini-games, like target practice and bowling, using the game's punching mechanics.