The Abominable Snowman
In the Himalayas in 1935 the Great Intelligence controlled the minds of a group of Buddhist monks and made them build the robotic yeti. The yeti are controlled by small control spheres inside of them. They were later encountered by the Second Doctor, Jamie, Victoria, and Professor Travers. When the pyramid of control spheres was smashed by Jamie all the yeti were deactivated and the Intelligence was dispelled.
The Web of Fear
One yeti and a control sphere survived and was kept by Travers, however the Intelligence used it to return. The yeti was used to make another army of taller slimmer yetis and the Intelligence now became a large poison web. He then sent the yeti to invade England where they waged war against the military led by Colonel Lethbridge Stewart. However he along with the Second Doctor, Jamie, and Victoria.
The War Games
During the Second Doctor's trial the Yeti were used as an example of the many things he saved the universe from.
The Five Doctors
The Second Doctor and the Brigadier encountered a Yeti in a cave in the Death Zone on Gallifrey. It attacked them, however the Doctor used a firework to distract it and cave it in.