From the TARDIS wiki:The Zygons were a race of metamorphic humanoids. They originated from the planet Zygor, but often tried to migrate away from it. ("Terror of the Zygons")
The Zygons were also accompanied by an armoured cyborg creature called the Skarasen, the lactic fluid of which was necessary for them to feed. Broton planned to unleash the Skarasen (or Loch Ness Monster as it was known) on an energy conference in London as part of a bid to conquer the Earth.
The plan was foiled and both he and his crew were killed due to the intervention of the Fourth Doctor and the United Nations Intelligence Taskforce (UNIT), while the Doctor caused the Skarasen to return to Loch Ness. Zygon technology is biological in nature: in essence their ships and equipment are actually alive.
Although they only figured in the one serial so far plus the 50th anniversary special, (mentioned in several other episodes, however), the Zygons are a key reason why 'Doctor Who' must be considered one of the "Essentials" - those TV shows which are vital to the existence of the Toobworld Dynamic.
Have you ever watched a TV show and thought to yourself that a certain character whom you knew so well was no longer acting the way they should? ("So and So would never do that or say that!")
If so, we could use the Zygons as the splainin. The theory would be that the character, long established to act a certain way, was now a Zygon replicant with an ulterior motive. And that would be why they were now acting differently.
The best part of all is the fact that they needed their test subjects to remain alive in order to maintain the illusion. So should the character finally return to the way he or she was supposed to be behaving, we could say that the original subject had escaped from the Zygons and returned to their life. All done off-screen, of course.
The real reason the character's personality or behavior changed could probably be chalked up to a change in the writing staff, but that's behind the scenes. Isn't this splainin more entertaining?