A sort of high-altitude troll adapted to living on very high mountains. Spins a thick fleece of extruded silicon fibres for warmth and bulk. First encountered in Moving Pictures when the distantly-heard Big Eats turn out to be a thousand indigestible elephants. On skis and sleds.
In Thief of Time we discover the species has an ability to manipulate time, so presumably the two specimens encountered in Moving Pictures learnt from what would otherwise have been a terminal encounter with a thousand elephants on bobsleighs - presumably they were able to pause and rewind, to a point where they could decide to be in a place where a thousand elephants on bobsleighs weren't. The Yeti's time manipulation is quite similar to the "save game" feature of video games on Earth. The Yeti is able to save its time at a certain point, and then venture forth knowing that if it dies, it can just resume its life from the point it saved at with the knowledge it acquired before death. It is effectively a highly evolved, albeit slightly painful form of foretelling. It is possible- as demonstrated by Lu-Tze- to learn how to do this without evolving it naturally, but given the difficulty involved in this method of survival when it isn't built in the person attempting it would have to be truly desperate to attempt such an action.
They are hunted for their feet, which as everyone knows have aphrodisiac qualities, as big feet mean other extremities are also oversized. Well-known fact, that is.
The Yeti, on Roundworld, is an example of cryptozoology, a large man-like creature reputed to inhabit the high places of the Himalayas. Often seen, but only by unreliable people with no zoological training, like local natives, and mountaineers who must evidently be running out of oxygen and hallucinating.
The Yeti has a North American cousin, the Sasquatch (aka Bigfoot)....