The Iwi is a tribe that inhabits Skull Island.
Origins
As revealed in Skull Island: The Birth of Kong, the Iwi were originally not native to Skull Island, as their ancestors migrated to the island thousands of years ago. They arrived in time to witness Kong's parents towering above dead Skullcrawlers.
Culture
Originally, the Iwi had no formal language, and preferred to communicate with each other through gesturing and facial expressions; however, as revealed by Ato, during the events of Kong: Skull Island, Hank Marlow taught his father English, and his father taught it to him, in turn. This allowed the Iwi to evolve by developing their own language.
They also worship Kong and his species as their gods and saviors, and fear the Skullcrawlers to the point of refusing to speak their true names.