Tree Dwellers (Commonthroat <qgKq-dq-g> /huff, early rising strengthening growl, huff, short high weak whine, huff, short low weak growl/ literally “forest yinrih”) are a non-sapient species closely related to the yinrih. While humans often compare them to chimps, they're more closely related to the yinrih than chimps are to humans. It's better to think of them like early hominins. They're clever, can use simple tools, and have complex hierarchical social structures, but do not conduct rituals or possess language, which are the criteria the yinrih use to determine whether something is a sophont.
The Tree Dwellers and yinrih evolved from a single population of animals that lived in an equatorial rain forest river basin. Over time, the river widened, isolating the two populations. The northern population became the tree dwellers while the southern population became the yinrih. Both species are arboreal, but can also navigate on the ground well enough. The basin floods annually, and both species would spend the dry season on the ground and in trees, while remaining in the trees during the wet season.
To humans they look _very_ similar to the yinrih. Their vocalizations even sound like yinrih speech, at least if you don't know Commonthroat or some other yinrih language. The only obvious visual difference is that Tree dwellers have more rounded ears (think the ears of a corgi vs the ears of a wolf or a German shepherd). Yinrih can smell the difference pretty easily, as well as spot other subtle visual cues in how they look and move.
The Tree Dwellers are similar enough to the yinrih that they're used for organ transplants and lab testing. In fact, before yinrih zoology was well developed, the yinrih considered themselves to be _literally_ sapient tree dwellers, with the only difference being that yinrih had rational souls and tree dwellers had animal souls. A (relatively minor) religious controversy arises when it's discovered that they are in fact different species.
The yinrih are more comfortable with their animal origins compared to humans. Since they evolved a written language, their history reaches back to the dawn of sapience in their species. The highest holy day for the Bright Way (before First Contact) is the feast of the kindling of the Fire of Understanding, which celebrates the emergence of sapience. Discovering evolution and genetics (other than the above-mentioned minor issue) is not seen as an impediment to faith. The yinrih knew all along they came from other animals, they just didn't know by what mechanism. This causes some friction with more pious humans. The yinrih don't understand why some humans could be uncomfortable with our status as great apes, but once the cultural particulars are explained, they are more sympathetic, even if they disagree. They may even refer to humans as apes, matter-of-factly or even out of affection, but never out of scorn or ridicule, which is only fair, seeing as how we often call them monkey foxes ourselves.
Wealthy yinrih may attempt to keep tree dwellers as pets. They're pretty cute as pups, but become aggressive when they mature. While they don't have the pound-for-pound strength advantage over yinrih that chimps have over humans, they're still more than capable of messing you up.
There's an urban legend that a group of Partisan scientists tried to teach a tree-dweller the concept of mortality. The outcome differs depending on who's telling the story.