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 eye-holes and flattened teeth.

Gizou had the ability to change both 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. (IDW: Agent Provocateur)

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 Doctor and Martha happened to have been in the same place. After the Sycorax killed the Gizou, he then went after the Doctor as the last of the Time Lords. (IDW: Agent Provocateur)

One Gizou assassin posed as "Lucas Finch", under which guise he served as prosecutor during the 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. (IDW: Fugitive)