Gizou

The Gizou were a species of shapeshifters.

Biology
In their natural form, Gizou were thin humanoids with purple skin, lumpy heads, and four digits on each hand. Their skulls were smaller than a human's, with larger eyeholes and flattened teeth.

Gizou could change their size and shape, allowing them to mimic other forms of life. This ability was linked to the proteus particles found in their body. They weren't entirely accurate, though, and made mistakes. (COMIC: Agent Provocateur)

The Gizou were a single-hearted species, a trait which their shapeshifting ability was unable to hide, allowing the Eleventh Doctor to realise that the Gizou disguised as Time Lords were frauds due to their lack of a binary vascular system. (COMIC: Time Fraud)

History
The last member of the Astrophia Tribe used Gizou in his business. Since he dealt with the hunting of nearly-extinct species, he could use the Gizou to extend the hunt by pretending to be the last members. Many would escape, though, meaning the Sycorax had to hunt them down.

When one escaped to Korova Moo, the Tenth Doctor and Martha happened to be in the same place. After the Sycorax killed the Gizou, he went after the Doctor as the last of the Time Lords. (COMIC: Agent Provocateur)

One Gizou assassin, Lau'Tan, posed as "Lucas Finch", under which guise he served as prosecutor during the Tenth Doctor's trial by the Shadow Proclamation. He was later exposed, but it was not revealed that he was working with the Doctor's Advocate. (COMIC: Fugitive)

During an attack on the TARDIS, a Gizou impersonated a Acari. (COMIC: Tesseract)

A group of Gizou mercenaries that included Nimu and Kemos were coaxed into working for the rogue Time Agent Scott Thrower by a promise of a cure to a deadly plague that was in reality caused by Thrower himself. The Gizou and their employer impersonated Time Lords on Helion, as Thrower tried to use the planet's solstice to rejuvenate himself. (COMIC: Time Fraud)