Inklings are a race of human-squid hybrids who inhabited Earth before it was destroyed by Tabuu. They killed all humans and took over by the year 2040. By 2080 they were ruled by DJ Octavio, who was pretending to be an Inkling but he was killed by Splat Tim. They live in a single city: Inkopolis in a country called Inkland.
They are an evolution of humans who evolved to survive in water. Due to this they felt themselves superior to humans so they genocided them all and took over Earth.
Sometime by the year 2080, a dictator named DJ Octavio became the ruler of the Inklings, pretending to be one of them by wearing a trench coat. He ruled with an iron fist, enslaving them and putting some of their children into a series of death games called the Splat Games. This ended up saving the inklings since they were able to assemble together and kill DJ Octavio. This was moments before Earth got destroyed by Tabuu.