From : Unknown
"Run...........! It's the yeti...........!" Professor Travers [Web Of Fear - 1968]
The Yeti, although humanoid in appearance masked a robotic internal skeleton mechanism which was controlled by a detachable small sphere.
The Yeti's aren't intelligent themselves in the sense of reasoning abilities, they are capable of simple tasks and their reasoning skills are basic at best. Their strength comes from their frightening appearance, almost indestructible shell and their brute strength
The internal control sphere, is itself controlled by the 'Great Inteliigence'. Little is known of the 'Great Intelligence' other than it was some kind of disembodied entity from another dimension, with designs on subjugating Earth.
The Yeti's are pretty indestructable repelling bullets, explosions and their only sign of weakness is their is control sphere, if it can be renched from the Yeti's frame, the Yeti then deactivates.
The Second Doctor first meets the Yeti in Abominable Snowman - 1967 when he travels to Tibet. The Yeti have been harrassing the some explorers and the local monks in the Monastry. At first, they accuse the Doctor of the attacks (as he is dressed in his fur coat) but soon come to realise there is something more frightening lumbering the harsh Tibetan hillsides.
Finally, it becomes apparent the robotic Yeti's are being controlled by the leader of the Monastry, who in turn is being controlled by the Great Intelligence. The Doctor is able to stop the Great Intelligence's influence, by destorying the equipment that controls the Yeti's sphere's.
Their next appearance was Web Of Fear - 1968, which also featured a first appearance of Lethbridge Stewart and Unit. The Great Intelligence and their Yeti servants were back, this time menacing the London Underground. The Yeti's were spreading a sticky web like fungus, which the Great Intelligence were using to try to wipe out the humans.
The Doctor this time thwarts the Great Inteliigence by reprogramming one of the Yeti's under his control and uses it to smash the relay the Great Inteliigence is using to manifest itself on Earth.
A lone Yeti next made a final appearance in The Five Doctor's - 1983 chasing the Second Doctor and Lethbridge Stewart through the Deadzone in the Games Of Rassilon. The Doctor held up in a cave, managed to scare the Yeti away with a small explosive device he refers to as a "cherry bomb".