Yetis are unintelligent robots that are controlled by the Great Intelligence, a being of pure thought from another dimension. They are its tools on Earth, carrying out physical tasks that either the intelligence or its human agents cannot manage.
Originally constructed in the Himalayas, the Yetis were given the form of abominable snowmen. They are about 2.1 metres tall, bipedal and covered in long shaggy brown fur. Their red eyes, black leathery snout and yellow fangs are barely visible through the mass of fur. Yetis look almost comically squat because of the breadth of their bodies, yet they are dangerous: their hairy hands end in razor-sharp talons, which the Yetis usually slash violently at an enemy. The creatures have a terrifyingly loud roar.
The Great Intelligence
Exiled from another dimension, the Great Intelligence drifts through time and space, seeking entry into the world of men. It requires a mental focus in order to materialize: this focus can be the mind of an oriental mystic caught wandering in the astral plane, or even an active relic of the intelligence's previous visitations, such as a Yeti control sphere. If its focus is destroyed before it can build up sufficient power to enter the physical world, the intelligence is temporarily banished. It is theoretically possible to destroy the intelligence in psychic conflict, but even the Doctor's mind is too weak to contemplate such an action.
Once the intelligence has a human agent, which in effect it hypnotizes and then possesses, it can gather together the materials necessary to build its robot servants, the Yetis, and their control spheres. A pyramid of control spheres is sufficient a focus for the intelligence to channel itself through. Its prime human agent gains the mental abilities of the intelligence.