Zygons are refugees from a stellar explosion that destroyed their home world of Zygos. A Zygon's manlike body is upright and bipedal; its green and orange skin, however, looks unpleasantly rubbery and is slimy to the touch. A great dome-like head meets the shoulders direct; there is no trace of a neck. Lines of sucker-like nodules run along the limbs and spine, and form a crest on the Zygon's head; a Zygon's fingertips end in small suckers. Deep-set, small eyes, a pug nose and a slight, puckered mouth give the Zygons an air of brooding menace.

Their burbling, sibilant voices sound menacing and betray the fact that Zygons are arrogant. Confident in their own superiority over other species, the Zygons seek new worlds to make habitable: they prefer warm wet climates. Because they are desperate to survive, Zygons will use any tactics they feel are necessary, even going as far as committing genocide. The Doctor had to deal with a Zygon spaceship and crew on Earth in Terror of the Zygons, when the Zygons planned to modify the Earth's climate to make it more suitable for their species and its herds of Skarasens, amphibious dinosaur-like creatures whose lactic milk is a vital component of the Zygons' life cycle. Zygon spaceships usually carry tanks containing Skarasen embryos.

Zygon technology uses organic materials, with the result that it resembles the creatures themselves: consoles have rubbery nodules as controls, and connections are usually in the form of suckers on long rubbery stalks. Even a Zygon spaceship is partly organic. Zygons have developed a body transference machine that allows them to adopt the physical form of any creature, provided that they have recorded its body print. The usual procedure is to capture suitable specimens, preferably including key political figures, record their body prints and allow shapechanged Zygons to take their places. The specimens need to be held onboard the Zygon spaceship in the event that their body print fades.