The Yeti robots were large and hairy to disguise the control spheres that provided their motive power. The sphere was hidden under a flap at the chest. Their hands, feet and eyes were the only parts not covered in fur. The hands and feet were black and bumpy. There were at least two iterations of robot Yeti. The ones encountered by the Doctor in Tibet were markedly bigger than the ones he encountered in England. Also, the ones in London had better-defined hands, capable of wielding guns. The Doctor called the ones encountered in London "a sort of Mark II".
The Yeti served the Great Intelligence, a disembodied entity from another planet, which tried to form a physical body in order to conquer the Earth in 1935. The Yeti were initially a ruse to scare off curiosity seekers; later, they became more of an army serving the Great Intelligence. They were similar in appearance to what Edward Travers called "real" Yeti.
More than forty years later, non-functioning Robot Yeti on display in London awoke and took over London. This event helped give impetus to the formation of UNIT, whose British arm was originally led by Alistair Gordon Lethbridge-Stewart.
Later, Lethbridge-Stewart would again encounter a Yeti on Gallifrey, left over from Rassilon's game. On this occasion, the Doctor maddened it with a firework, and the resulting rock fall prevented any further interaction with it.