Great Intelligence

The Great Intelligence (or Yog-Sothoth) has no physical form. As a disembodied sentience, it seeks bodies and physical existence and has used the Robot Yeti to accomplish this.

History
Yog-Sothoth was one of the Old Ones who existed before the present universe (MA: Millennial Rites). In the present universe, the Tibetan lama Padmasambhava on the astral plane. The Great Intelligence possessed Padmasambhava. In 1935, the Doctor, a friend of the lama's, as well as Edward Travers, a westerner determined to find the Yeti, intervened. Padmasambhava's physical body died, already essentially dead when the Intelligence had possessed him, melted away. (DW: The Abominable Snowmen)

Thirty-five years or so later, the robot Yeti re-activated and the Intelligence manifested as webbing, ensnaring the Doctor's TARDIS in space and forcing it to land in the London Underground, where, reunited with Travers, the Doctor assisted British military in their battles with the Yeti. The Intelligence re-animated and possessed the corpse of Staff Sergeant Arnold. The Inteligence captured the Doctor, attempting to use a conversion headset to take over and possess the Doctor's body, a process the Doctor had ensured would reverse so that the Doctor would have absorbed the Intelligence. With the control spheres that formed the focus of the Intelligence smashed by the Doctor's companion, Jamie McCrimmon, the Intelligence vanished before this happened so that the Intelligence still existed in some form. DW: The Web of Fear)

The Intelligence later contacted the Doctor's former companion Victoria Waterfield and manipulated her into using computers to return to physical existence. (MA: Downtime)

Physical existence
The Intelligence used reflective glass control spheres to take the form of a pyramid of the spheres and formed a locus for the Intelligence's influence. (DW: The Web of Fear) Also, it manifested as webbing in order to trap the Doctor's TARDIS. (DW: The Web of Fear)