Zygon

The Zygons were a race of metamorphic humanoids. They originated from the planet Zygor, but often tried to migrate away from it. (PROSE: The Bodysnatchers)

Biology
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 apparently more long-lived, their lifespan continuing over Earth centuries. (TV: 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. (PROSE: Sting of the Zygons, PROSE: Doctor Who and the Loch Ness Monster) They also had the ability to create a process to transform members of other species into Zygons, with the side effect of creating a telepathic link between the new Zygon and the one that induced the change. (HOMEVID: Zygon: When Being You Just Isn't Enough)

They had venom sacs located in their tongues. (TV: The Day of the Doctor)

Young Zygons were called hatchlings and were slimy and pale with shorter limbs. Hatchlings were mute, but much faster than adults. (PROSE: Sting of the Zygons) Zygons had a deeply ingrained fear of fire. (PROSE: Doctor Who and the Loch Ness Monster) They could be easily hypnotised by a clock's swinging pendulum. (GAME: Destiny of the Doctors)

Zygons were hermaphrodites that could reproduce asexually. At the start of their lives, they had an almost white colour, had no suckers and were more feminine and graceful. They became orange-coloured, were covered in suckers and became more stocky and masculine in appearance after being sterilised. The greatest rank a Zygon could attain was "warrior engineer". To become a warrior engineer any Zygon must undergo sterilisation, a process which makes them more aggressive and single-minded. (PROSE: The Bodysnatchers)

Zygons were able to produce electricity bolts from their hands that could stun humans or other Zygons or disintegrate them into piles of hair and skin. When used on the Twelfth Doctor, however, he was slightly phased, but otherwise suffered no ill effects. (TV: The Zygon Invasion)

Technology
Zygon technology was partly biological, with an organic look. This was exemplified by the Skarasen, a large, cybernetically enhanced creature from their home world. Zygons were also known to depend upon the lactic fluid of the Skarasen for their survival. (TV: Terror of the Zygons) Their use of organic ships was well known. (PROSE: Alien Bodies)

One of the most important Zygon discoveries was their use of body-prints. Usually, 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. (TV: Terror of the Zygons) However, in some circumstances they were able to adopt the shape of the specimen without keeping them in a receptacle, although they still needed the specimen alive. (TV: The Day of the Doctor) At some point they gained the ability to keep the shape of a deceased specimen, as long as they did not need any more information from them. They also gained the ability to forge a telepathic link with another being and take the form of a someone from their memory. (TV: The Zygon Invasion)

Though they generally took the forms of humanoids, they could also mimic creatures of diverse sizes and shapes, such as dogs, cattle and horses. (PROSE: Sting of the Zygons, TV: The Day of the Doctor) A Zygon who could no longer transform was known as a Zynog. (AUDIO: Death in Blackpool) If a Zygon was killed while disguised, it would remain in that form as a corpse. (HOMEVID: Zygon: When Being You Just Isn't Enough)

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. (TV: Terror of the Zygons, PROSE: Sting of the Zygons) They also made use of a type of nerve gas to render humans unconscious. (TV: Terror of the Zygons) Another Zygon discovery was called the "amber sleep", which was a type of suspended animation. (PROSE: Sting of the Zygons)

Culture
Zygons were divided into clans, each with a different culture. (AUDIO: The Zygon Who Fell to Earth) Zygon warlords fought in elaborate shape-shifting duels using the bodies of other creatures held captive. (COMIC: Rest and Re-Creation) A Zygon High Command organised Operation Double on Earth with the Unified Intelligence Taskforce before being violently overthrown and replaced by a faction of radical Zygons called Truth or Consequences which sought to usurp humanity. (TV: The Zygon Invasion)

Zygons measured distance in remars. (TV: Terror of the Zygons, PROSE: Sting of the Zygons)

Zygons were evidently rather fond of board games. (PROSE: The Bodysnatchers)

Early history
The Zygons joined Rassilon in the Alliance of Races against the Hyperions. (COMIC: The Hyperion Empire) After the Hyperions were defeated, the Alliance then went on to purge the universe of other threats to universal harmony, one of those threats being Count D'if of the Cybock Imperium. (COMIC: Gangland)

Loss of Zygor
According to one account, the Zygons were originally at war with the Xaranti. During this war, both the Zygon homeworld, Zygor, and the Xaranti home planets were destroyed. (PROSE: The Bodysnatchers) Another account claimed that their planet burnt in the first days of the Last Great Time War. (TV: The Day of the Doctor)

Refugees created a refugee fleet, and the ship on Earth was contacted. (TV: Terror of the Zygons)

2nd century
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. (TV: The Pandorica Opens)

Before the 12th century
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. (TV: Terror of the Zygons)

16th century
In 1562, a Zygon force was on Earth and investigated by the Tenth Doctor, one of them impersonating Elizabeth I. After the real Elizabeth killed her impersonator, the Zygons implanted themselves into stasis cubes, planning to wait until Earth was more suitable for conquest. (TV: The Day of the Doctor)

19th century
In the late 19th century, a Zygon became a member of a travelling freak show and later impersonated Queen Victoria. (AUDIO: The Barnacled Baby)

A Zygon ship was trapped under where the Savoy Hotel was. During the grand opening in 1890, Zygons replaced half the staff, but the Eleventh Doctor, Amy Pond and Rory Williams stopped them. (TV: The Power of Three)

The Eighth Doctor, Sam Jones and George Litefoot encountered a group of Zygons in London in 1894. (PROSE: The Bodysnatchers)

20th century
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. (PROSE: Sting of the Zygons)

In the 20th century, a Zygon spaceship landed in a Brentford quarry. 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. (TV: Terror of the Zygons)

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

In 1984, the Eighth Doctor and Lucie Miller encountered a Zygon who was impersonating Lucie's aunt, Patricia Ryder. (AUDIO: The Zygon Who Fell to Earth)

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. (COMIC: Skywatch-7)

Circa 1999, a Zygon invasion of Earth was prevented by UN military advisor Guy Dean. (AUDIO: Homeland)

21st century
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 respectively. They then began to go to nuclear power plants around the world, so that 70 years later the planet would be more suited to Zygons 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 killed many of her co-workers at the hospital. She left, but not until after she killed Torlakh while he was in her form. (HOMEVID: Zygon: When Being You Just Isn't Enough)

In 2008, the Eighth Doctor and Lucie Miller again encountered the Zygon who impersonated Patricia Ryder. (AUDIO: Death in Blackpool)

In the early 21st century, the Zygons that were frozen in 1562 woke from their stasis cubes and infiltrated UNIT, gaining access to the technology in the Black Archive. To prevent them using the technology, Kate Stewart activated the countdown to a nuclear warhead beneath the Archive. Before the countdown reached zero, the Tenth, Eleventh and War Doctors temporarily erased the memories of the original Kate, Osgood and McGillop, as well as their Zygon impersonators causing them to forget which ones were human and which were Zygon. Both Kates cancelled the detonation, and the humans and Zygons were able to create a peace treaty between them. When no one was looking the Zygon copy of Osgood slipped the original's inhaler back to her. (TV: The Day of the Doctor) Two years later, a rebel faction of Zygons, led by Bonnie, began to undermine the ceasefire. Bonnie had taken the form of Clara Oswald, and was determined to expose the 20 million Zygons living on Earth and find a land of their own, (TV: The Zygon Invasion) but this was defused by a contingency developed by the Twelfth Doctor involving two Osgood Boxes. (TV: The Zygon Inversion)

22nd century
150 years after Kritakh and Torlakh were brought to Earth, the Zygon invasion force was still due to arrive on Earth. (HOMEVID: Zygon: When Being You Just Isn't Enough)

28th century
In 2765, Zygons attempted to melt Earth's polar icecaps, but were stopped by INITEC's Vigilante laser defence satellites. (PROSE: Original Sin)

36th century
Circa the early 36th century, a group of Zygons on Ganta 4 infiltrated the New Eden colony so they could use their spaceship to evade Earth security and invade humanity's homeworld. The plan was defeated by colonist Christopher Shaw, and the Zygons were all killed. (AUDIO: Absolution)

Undated events
On Shontaa, the Zygon warlords Anktra and Kestral fought a duel using a variety of host bodies. The Fourth Doctor and Leela ended the duel by freeing the host creatures which the Zygons were mimicking. (COMIC: Rest and Re-Creation)

, 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. (GAME: Destiny of the Doctors)

The Divine Empress had Zygons under her command. (PROSE: Genocide)

River Song once angered Zygons by greeting them with, "What's up, suckers?" (GAME: The Eternity Clock)

A yellow Zygon was present at Garage 10. (COMIC: The Ministry of Time)

The Ninth Doctor defeated a Zygon who took the form of Rose's grandmother. He then disabled it's body-print technology. (PROSE: Little Rose Riding Hood)

Alternate timelines
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 against the Silurians and Sea Devils. This timeline was negated when the Seventh Doctor retrieved the Key to Time. (COMIC: Time & Time Again)

In an alternate dimension in which the Doctor's adventures existed as a fictional TV show called Doctor Who, a fan dressed as a Zygon was present at a Doctor Who convention which the Eleventh Doctor visited with Ally in 2013. (COMIC: The Girl Who Loved Doctor Who)

Behind the scenes

 * The prisoners of the Time Lord prison Shada may have included a Zygon. (HOMEVID: Shada)
 * Zygons are used as both allies and enemies in Doctor Who Legacy mobile game. In addition, many enemies are created to be Zygons taking the form of an ally.