The first two MOTHER games (Earthbound Beginnings & Earthbound) take places in the 1900’s, while MOTHER 3 takes place in a post-apocalyptic future.
The Splatoon series takes place at least 12,000 years after the extinction of most land animals.
MOTHER 3 may be the closest to Splatoon’s timeline (since humans were near extinction), but the few sea creatures were not even close to humanoids. Perhaps the reason why sea animals evolved so quickly is because of the chimera experiments?