They are the inhabitants of Skull Island who are the only known humans to live there.
The mystery of the ruins.
The first sight to greet the Skull Island explorers was the powerful wall that separated the poor islander settlement from the terrors of the jungle interior. Huge and imposing, this massive structure surpassed the fragile village that nestled in its protective shadow. This was not the work of the human population that inhabited the island when SS Venture arrived on the skull island, but the legacy of some older and much more advanced civilization that long ago disappeared. Gigantic stone ruins of this ancient society were scattered throughout the island. Large buildings jutted out of the jungle and down the coast to disappear into the sea. Beneath the tangled forest that enveloped the island in a suffocating green embrace, a great city had once breathed. The study of these remains and the great wall itself, which operated in an uninterrupted circle around the entire central part of the island, spoke of a culture that was three thousand years old. Architectural parallels suggested Southeast Asia as the origin of this civilization.
It was theorized that the ancient settlers brought with them an established culture, as evidenced by the large carved statues and the splendid ceramic fragments that remained. They were a dedicated culture that revered the giant primates that abounded in all their art. Some give the theory that the apes may have arrived with the settlers, alluding to a symbiosis between the people and Kong's ancestors. In fact this theory is more credible since other 2 animals that were brought by this civilization have prospered on the island, those are the Gaur bovine which is a subspecies of the cattle that inhabit continental Asia and the Bloodfish that are descendants of the cyprinids and which have been so successful that they have managed to fill almost all niches on the island, causing the extinction of native lungfish, except for piranhadon and panderichthys.
The exact nature of the extinction that affected these people remains a mystery. At least a thousand years ago, they were annihilated, most likely due to the reduction of the island that caused a collapse of its monuments, its territory and that the creatures of the island began to enter its territories, in the end civilization disappeared in history, and the wall that surrounded what was a huge city was almost completely destroyed.