Zygon

The Zygons were a race of metamorphic humanoids.

Physical characteristics
Zygons are solidly built humanoids with large, cone-shaped heads. Their heads, arms and torsos are covered in suckers and they have deeply inset faces. Zygons have dark red blood and speak in a gurgling whisper. Zygons are stronger then humans and can live for several centuries. (DW: Terror of the Zygons) They have an earthy, iron smell. Zygons have the ability to sting people using venomous barbs on their palms. These stings can be used to stun, maim or kill, and will leave severe welts on the affected area. (TDA: Sting of the Zygons, DW: Doctor Who and the Loch Ness Monster)

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

Zygons also have a very deeply ingrained fear of fire. (DW: Doctor Who and the Loch Ness Monster)

Technology
Zygon technology is partly biological and all their technology has a half-grown look. This is shown by the Skarasen, a large creature from Zygor which has been cybernetically enhanced.

One of the most important device of the Zygon is their shapshifting ability through body-print technology. The Zygons can capture a Human and put them in special receptacles. They can then take the form of the human, but they need periodic bio-scan checks in order to maintain the illusion. Because of this, the subjects are kept alive but unconscious during the whole process. This process copied both the appearance and the voice of the subject, though the Zygon's actions may give them away. (DW: Terror of the Zygons) Though they generally take the forms of humanoids, they can become creatures of digfferent sizes and shapes, such as dogs and cattle. (TDA: Sting of the Zygons)

Zygons can also use molecular dispersal to get rid of corpses. Their ships track the life-signs of the Zygons and will bring the body back to the ship if the Zygon is killed. Another technology is called the "amber sleep", which is a type of cryogenic sleep. (TDA: Sting of the Zygons)

Culture
Little is known about the Zygon culture, since the only Zygons shown were soldiers. Zygon warlords have been seen fighting elaborate shape-shifting duels using the bodies of other creatures held captive. (DWA: Rest and Re-Creation)

Zygons measure sound in remars. (DW: Doctor Who and the Loch Ness Monster, TDA: Sting of the Zygons)

History
The Zygons were originally at war with the Xaranti. During this war, both the Zygon homeworld, Zygor and the Xaranti home planets were destroyed. (PDA: The Bodysnatchers). Refugees were able to create a refugee fleet. One of the war-ships was damaged and eventually made it to Earth, where the Zygons hid in the Devil's Punchbowl in Scotland since at least the 12th century. They subsisted on the milk of the Skarasen, which they had brought in embryo form from their home planet and converted into a cyborg, which the Zygons controlled. Sightings of the Skarasen would inspire legends of the Loch Ness Monster. (DW: Terror of the Zygons)

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 and 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 Earth. (TDA: Sting of the Zygons)

In the late 20th century, the Zygons of Devil's Punchbowl learned that Zygor was destroyed and they decided to take over Earth and physically adapt it to an environment more to their liking. UNIT and the Doctor stepped in, though in the meantime the Skarasen swam south from Scotland to London via the Thames. The status of the Zygon invasion fleet still headed to Earth remained unresolved, though by implication, it would take some time to arrive. (DW: Terror of the Zygons)