The plainsriders are the barbarians: the Huturl tribes, the earlier Ular tribes, comprising both humans, hobgoblins, and other semi-nomadic, warlike races.
The Illaesians are the peoples from the northwestern islands: humans, faeriefolk, and wild elves.
The Dagean natives are those races and peoples who were living in mainland Dage when the invasions began: humans, halflings, gray and high elves, hill dwarves, gnomes, ogres, goblins, and snakekin.