Monkey-like sentient creatures of Channelwood. They built villages of huts half way up the grand trees, 10 meters above the ground.
When their island was about to collapse, legends say that the Channelwood humans sacrificed themselves to stop it. This is most likely the reason that they revered humans as gods or heroes. Even then, they died out. The last survivor was revered and was sitting next to their leader's throne, and were expecting the coming of someone like him.
When Atrus linked to that Age, the tree-dwellers recognized him as the one that they were sure that would come and started worshiping him, especially after the last human fell to his death, and started offering him food and objects of affection. They were also enthusiastic when they met his sons, Sirrus and Achenar. Atrus warned them not to take advantage of the reverence towards them.
When the brothers grew up evil, Channelwood was one of the Ages they plundered tyrannically, and they established themselves in palaces among the highest levels of the trees. It is obvious that Achenar played as a god and used an Imager to command the creatures, whom he also tortured or slaughtered.
The tree-dwellers spoke a language of their own which Atrus could not understand but later started to learn; his sons managed to pick it up faster than he. The Imager messages made by Achenar and used to deceive the tree-dwellers are obviously in that language.
Their leader lived in an enormous hut garnished with bright metals. He was dressed in many exotic, colorful fabrics.
The tree-dwellers apparently had evolved some sort of alchemy. They cremated the body of the old man by employing a potion which lit fire, and created a variety of inks for Atrus to use in his time in Channelwood.
They also had a sense of humor: as a practical joke, they insisted that Atrus use their inks for his journal; months later he discovered that it changed various colors, and they laughed and howled when Atrus asked for an explanation.
There is a stuffed ape head adorned in the Mechanical fortress; it is possible that it's a severed head of a tree-dweller, as indicated by the bloody chopping block and cleaver in Achenar's cache, as well as the fact that they slaughtered all of the inhabitants of the ages.