Robot Yeti

Robot Yeti were devices created by the Great Intelligence. They may have been based on living Tibetan Yeti. However, because no living Yeti were ever definitively captured and studied, it was also possible that the robotic Yeti was the source of the myth.

Appearance
The Yeti robots were large and hairy to disguise the control spheres that provided their motive power. The spheres were 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. 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". (DW: The Abominable Snowmen, The Web of Fear)
 * Because no clear pictures exist of the supposed "real" Yeti seen at the end of The Abominable Snowmen, direct comparison of the Mark I and "real" Yetis, as televised, is impossible. However the novelisation does afford such a comparison: "It was taller and less bulky. The fur was longer and silkier, and had a more reddish tint. Above all, the face was different, rather like that of a lemur, with dark, soft eyes." Nevertheless, it is extremely unlikely that the brief wide shot that was included in the episode as broadcast would have been able to convey this level of detail. Indeed, this is an instance where the novelization may be said to actively contradict the televised episode. Since episode six is missing, it is impossible to judge the reliability of this description.

History
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. (DW: The Abominable Snowmen)

More than forty years later, non-functioning Robot Yeti on display in London awoke and took over London. (DW: The Web of Fear) This event gave impetus to the formation of UNIT, whose British arm was originally led by Alistair Gordon Lethbridge-Stewart.

Lethbridge-Stewart would again encounter a Yeti on Gallifrey, left over from Rassilon's game. On this occasion, the Second Doctor maddened it with a firework, and the resulting rock fall prevented any further interaction with it. (DW: The Five Doctors)

A group of Yetis were put in the Doctor's TARDIS by the Master to battle Graak. (VG: Destiny of the Doctors)

Behind the scenes

 * The date of the second Yeti invasion is the point at which the UNIT dating controversy begins. The script of episode two of The Web of Fear says that the events of Snowmen took place "in 1935" which is in turn said to be "over forty years ago". This means that "Web" has to take place no earlier than late 1975. Since Lethbridge-Stewart is only a colonel in Web, it means his next appearance in The Invasion, and all the subsequent ones must take place even later than 1975, well after Tom Baker had begun to play the Fourth Doctor in real life. However, episodes made under John Nathan-Turner, and perhaps particularly Mawdryn Undead, indicate that the UNIT stories all took place in more or less the year they were broadcast. Also, the semi-official UNIT website gives the date of the incident as 1968, the same year The Web of Fear aired. Hence, the Tenth Doctor eventually refers to his time working for UNIT as vaguely happening in the 1970s or 1980s. (DW: The Sontaran Stratagem)
 * The sound effect of a Yeti's roar was created by slowing down the sound of a flushing toilet.
 * SJA: Eye of the Gorgon includes a dialogue reference to a sultan who had seen a Yeti. It's not known if this refers to a "real" Yeti or the robotic version.