Zygons were solidly built humanoids with large, cone-shaped heads. Their heads, arms and torsos were covered in suckers, and they had deeply inset faces. Zygons had dark red blood and spoke in a gurgling whisper. Zygons were stronger than humans and could live for several centuries. They also had an earthy, iron smell. Zygons had the ability to sting people using venomous barbs on their palms. These stings could be used to stun, maim or kill, and would leave severe welts on the affected area.
Young Zygons were called hatchlings and were slimy and pale with shorter limbs. Hatchlings were mute, but much faster than adults.
Zygons had a very deeply ingrained fear of fire.
Zygon technology was partly biological in basis, giving it an organic look. This is exemplified by the Skarasen, a large creature from Zygor which was cybernetically enhanced.
One of the most important Zygon discoveries was their body-print technology. The Zygons would capture a specimen, such as a Human, and put them in a special receptacle. A Zygon could then take the form of this specimen, mimicking both voice and appearance. This body-print needed to be renewed every 2 hours, which meant the prisoners were kept alive but unconscious. Though they generally take the forms of humanoids, they could also mimic creatures of different sizes and shapes, such as dogs and cattle.
Zygons frequently used molecular dispersal to get rid of corpses. Their ships tracked the life-signs of the Zygons and would bring the body back to the ship if a Zygon was killed. They also made use of a type of nerve gas in order to render Humans unconscious. Another Zygon discovery was called the "amber sleep", which was a type of cryogenic stasis.
Little is known about Zygon culture, since only Zygon soldiers have been seen. Zygon warlords have been depicted fighting in elaborate shape-shifting duels using the bodies of other creatures held captive.
At some point before the 12th century, a Zygon ship was damaged and was forced to land on Earth near Devil's Punchbowl in Scotland. They had a Skarasen with them, which, though still an infant at that point, provided them with the lactic fluid they needed to survive.
The Zygons were originally at war with the Xaranti. During this war, both the Zygon homeworld, Zygor and the Xaranti home planets were destroyed. Refugees were able to create a refugee fleet, and the ship on Earth was contacted.
Another ship from Zygor arrived on Earth in 1909. Though they had brought 2 Skarasen, their minds were damaged by the solar radiation brought on during the destruction of Zygor. One was killed during the attempt to repair the damage. Though the Zygons attempted to conquer Earth anyway, due to starvation -- and the actions of the Doctor -- they were forced to abandon their plans.
In the late 20th century, the Zygons of Devil's Punchbowl decided to take over Earth and physically adapt it to an environment more attuned to their species. UNIT and the Doctor stepped in and were able to stop them, letting the Skarasen free at the same time.
The Graske also kidnapped a Zygon, implying there were Zygon Changelings.