Thoughts on Origins of the Hinterlanders
by Simone NeriDotE has it this way:
a) Before the Thyatians, "other races" had lived in the Hinterlands, and had migrated northward to Brun as ice caps receded after the GRoF;
b) Thyatians lived here in ancient times, then most of them migrated north to Brun;
c) Those who stayed in Davania were later absorbed by the "next wave of migrants", calling themselves the "Men of the Four Kingdoms".
"Other races" mentioned in (a) above could well be human tribes, as well as non-human or demihuman ones (halfling, or even more likely elves "from beyond the seas of Thanegioth", as GAZ3 has it).
So the Hinterlanders should be the result of the Thyatian-Men of the Four Kingdoms mix. Looking at their names, it seems their tongue is loosely related to RW Welsh or Irish tongues. From PWA2, we learn that the Hinterlanders' native language is called "Thratian".
In the HW boxed set, it is revealed that the ancestors of Thyatians, Kerendans, and Hattians of the ancient Hinterlands were Northman slaves brought by the Nithian Empire in its newly established (as per 1000 BC) colony in the modern Raven Scarp area; those slaves later rebel against the Nithians, "slay their Nithian overlords", and "carve out their own territory in the southern continent".
VotPA, Part 1, has Haldemar flying above the region, and adds some information:
The savages who live on the eastern Jungle Coast are quite different from those in the neighboring Thyatian Hinterlands. The latter are believed to be descendants of slaves brought from the Nithian colonies nowadays known as Ostland and Vestland. The Nithians carved out a southern domain from the jungle for their priests. Then three tribes of slaves rebelled and escaped north, seeking their fatherland. Instead, these ruffians found (and founded) what would later become Thyatis. A century later, Nithia foundered. The unruly slaves who stayed south obliterated whatever remained of their Nithian origins; in a few centuries, all was lost to the jungle. These hardy, blond Hinterlanders survived and became savage jungle warriors who were capable of fighting the original natives on equal footing.
Since the ancient Thyatians have a Northman ancestry, the M-Welsh/Irish/Gaelic character of the modern Hinterlanders should supposedly come from the "Men of the Four Kingdoms" who later conquered those members of the Three Tribes (Thyatians, Kerendans, and Hattians) who chose to stay behind. It seems however there were also some "original natives" - they could have been other Neathar tribes, or even older tribes descended from local Oltecs.
Something is off in this paragraph, however, because the "unryly slaves who stayed south" obliterating the last Nithian vestiges should apply to the members of the Three Tribes left in Davania after 600 BC - why did they wait to long (1000-600 BC) to destroy the last trapping of Nithian civilization? Why did they do this on the eve of their likely conquest by the Milenians? Maybe the sentence should apply instead to the whole group of the former Northman slaves after they threw off the Nithian joke after 1000 BC, but that's not what it literally says...
PWA2 and HW boxed set also reveal that the Milenian Empire expanded in the Hinterlands area around 600 BC, when most of the Three Tribes, hard-pressed, fled to Brun. The empire then collapsed in the 1st century BC. It's not clear in canon sources, but I'd say that the absorbption of the Thyatians who stayed behind by the "Men of the Four Kingdoms" happened after the collapse of the Milenian Empire (also because Vanya was a Kerendan princess of the time when the Three Tribes moved northward to Brun, and she I think it's safe to assume she was of fully "Thyatian/Kerendan" ethnicity).
So, as far as canon goes, the picture should be this one:
a) [Before 1000 BC] Native tribes of Neathar (or Oltec, or demihuman, or nonhuman) origin live in the Hinterlands;
b) [1000 BC] The Nithians transplant Northman slaves to Davania to serve them in their new colony; nearby, the original core of what would become the Milenian Empire is established by Traldar exhiles;
c) [Some times after 1000 BC] The Northman slaves rebel to the Nithians, destroy their colony, and found their own realm (who maybe preserves something - technology, architecture, infrastructures, whatever - of the Nithian dominion); in the intervening centuries, these Northmen likely come into contact with the Milenian culture and are influenced by it;
d) [600 BC] The Milenian Empire expands in the Hinterlands region; many of the Northmen - the Three Tribes (Hattians, Kerendans, and Thyatians) - flee northward to Brun, while others stay behind (the mortal princess later known as Immortal Vanya is among them) and are likely conquered by the Milenians;
d) [Between 200 and 50 BC] The gradual decline and collapse of the Milenian Empire frees the Hinterlands region from the Milenian control;
e) [After 50 BC] The descendants of the ancient Northman slaves are conquered and absorbed by the "Men of the Four Kingdoms" migrating there and establishing their own realms.
Regarding the ethnological picture, my interpretation is that a number of M-Gaelic, Neathar-descended peoples existed in the Hinterlands and northern Davania before the arrival of the Northman slaves brought by the Nithians. These M-Gaelic peoples include both part of the "original natives", and the later "Men of the Four Kingdoms", and should be related to the M-Gaelic peoples of Brun (Robrennians, likely many others both in canon and fanon) and the Isle of Dawn (Redstoners, at least, since they have M-Irish placenames in canon, as well as many others from fanon). The modern Hinterlanders (aka Thratians) should be the result of the fusion between an M-Gaelic people with the descendants of the Northmen who stayed in Davania.