It's always a mystery how an average cat named Mewrus became the mammalling cat they are today. Maybe it was accidental? Maybe curiosity got the better of them?

All that is known is that it took place in that weird metro below inkopolis.
(Background is a screenshot from the Octo Expansion DLC of Splatoon 2. This is also a canon hint at what potentially made Mewrus the mammalling cat they are today.)