Zygon

The Zygons were a race of metamorphic humanoids.

Physical characteristics
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. They were stronger than humans and could live for centuries. (DW: Terror of the Zygons) They also had an earthy, iron odour and the ability to sting people with venomous barbs on their palms. These stings could stun, maim or kill. They left large welts on the affected area. (NSA: Sting of the Zygons, DWN: Doctor Who and the Loch Ness Monster)

Young Zygons were called hatchlings and were slimy and pale with shorter limbs. Hatchlings were mute, but much faster than adults. (NSA: Sting of the Zygons)

Zygons had a deeply ingrained fear of fire. (DWN: Doctor Who and the Loch Ness Monster) They could be easily hypnotised by a clock's swinging pendulum. (VG: Destiny of the Doctors)

Technology
Zygon technology was partly biological, with an organic look. This was exemplified by the Skarasen, a large, cybernetically enhanced creature from Zygor. (DW: Terror of the Zygons) Their use of organic ships was well known. (EDA: Alien Bodies) 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 two hours, while the prisoners were kept alive but unconscious. (DW: Terror of the Zygons) Though they generally took the forms of humanoids, they could also mimic creatures of diverse sizes and shapes, such as dogs and cattle. (NSA: Sting of the Zygons) A Zygon who could no longer transform was known as a Zynog. (BFA: Death in Blackpool)

To control the Skarasen, the Zygons used Trilanic activators. Zygons frequently used molecular dispersal to get rid of corpses. Their ships tracked the life-signs of the Zygons and would bring the corpse back to the ship if a Zygon was killed. (DW: Terror of the Zygons, NSA: Sting of the Zygons) They also made use of a type of nerve gas to render humans unconscious. (DW: Terror of the Zygons) Another Zygon discovery was called the "amber sleep", which was a type of suspended animation. (NSA: Sting of the Zygons)

Culture
Zygons were divided into clans, each with a different culture. (BFA: The Zygon Who Fell to Earth) Zygon warlords fought in elaborate shape-shifting duels using the bodies of other creatures held captive. (DWY: Rest and Re-Creation)

Zygons measured distance in remars. (DW: Terror of the Zygons, NSA: Sting of the Zygons)

History
In 102, the Zygons were among the races who joined the Alliance at Stonehenge. They came to Stonehenge and helped imprison the Eleventh Doctor in the Pandorica to "save" the Universe. (DW: The Pandorica Opens)

At some point before the 12th century, a Zygon ship was damaged and 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. (DW: Terror of the Zygons)

The Zygons were originally at war with the Xaranti. During this war, both the Zygon homeworld, Zygor, and the Xaranti home planets were destroyed. (EDA: The Bodysnatchers) Refugees created a refugee fleet, and the ship on Earth was contacted. (DW: Terror of the Zygons)


 * When this destruction of Zygor occurred is unknown.

Another ship from Zygor arrived on Earth in 1909. Though they had brought two Skarasen, their minds were damaged by the solar radiation during the destruction of Zygor. One was killed during repairs. Though the Zygons attempted to conquer Earth anyway, starvation and the Tenth Doctor forced them to abandon their plans. (NSA: Sting of the Zygons)

In the late 20th century, the Zygons of Devil's Punchbowl decided to take over Earth and physically adapt it to an environment more suited to their species. UNIT and the Fourth Doctor stopped them, freeing the Skarasen. (DW: Terror of the Zygons)


 * The status of the Zygon invasion fleet still headed to Earth remained unresolved, though by implication, it would take some time to arrive.

In 1981, UNIT had to deal with over a hundred Zygons who survived a previous invasion. (PDA: The King of Terror)

In December 1985, a Zygon landed in Antarctica, near UNIT Arctic base Skywatch-7. Several people stationed there were killed, but the survivors were able to drive it into the icy water where it perished. (DWM: Skywatch-7)

In the late-20th and early 21st centuries, the Zygons Torlakh and Kritakh were deployed on Earth, where they took the forms of Bob Calhoun and Michael Kirkwood respectivley. There, they began to go to nuclear power plants around the world, so that 70 years later the planet would be more suited to Zygon's and they could hand the planet over to a Zygon invasion force. The plan backfired when Kritakh began to believe he was Michael Kirkwood, and began to see a psychiatrist about his dreams of being a Zygon. His doctor, Lauren Anderson, was turned into a Zygon by Torlakh, and she went to see him again. However, she decided to help Michael stop Torlakh. They put the real Bob Calhoun and Michael Kirkwood in the hospital, and Torlakh left to continue his life on the run. Lauren, however, also was forced to live on the run, when Torlakh kill many of her co-workers at the hospital. She left, but not until after she kill Torlakh while he was in her form.

70 years later, the Zygon invasion force was still due to arrive on Earth. (BBV: Zygon: When Being You Just Isn't Enough)

In 2765, the Zygon attempted to melt Earth's polar icecaps, but were stopped by INITEC's Vigilante laser defence satellites. (NA: Original Sin)

Undated events

 * The Master, using the power of the planet Siralos, transported some Zygons and other adversaries to the Doctor's TARDIS to stop the Graak from saving the imprisoned seven incarnations of the Doctor. (VG: Destiny of the Doctors)
 * The Divine Empress had Zygons under her command. (EDA: Genocide)

Alternate timeline
In an alternate timeline created by the Black Guardian where the First Doctor never left Gallifrey, and became Lord President, the Zygons were one of many aliens that invaded Earth, and fought over the planet with other races. This timeline was destroyed when the Seventh Doctor retrieved the Key to Time. (DWM: Time & Time Again)