According to Seita Inoue, during the development of Splatoon the characters started off as rabbits before becoming inklings.

"Well, a lot of it was because of design factors. Rabbits are white to begin with, so it would be easy to tell if they were inked. They've got long ears that would move when they did, so you could see them moving and when viewed from above, it would be easy to tell which direction they were facing by the direction of the ears."