- You may be looking for the Earth reptile.
The Chameleons were an advanced humanoid race who developed a technique for stealing the identities of others.
The Chameleons claimed to be the "most intelligent race in the universe" and that the Earth people were as intelligent as the animals of their world.
A "gigantic explosion" on the Chameleons' home planet resulted in the loss of their faces and their identities. To save their race from dying out, their scientists developed a process that would allow them to assume the identities of other beings. A group of Chameleons under the command of the Director went to Earth and established Chameleon Tours at Gatwick Airport as a front for abducting young humans to their space station.
By July 1966 they had acquired 50,000 people, but before they could return to their world they were exposed by the Second Doctor, who offered to help them find another solution to their problem in exchange for releasing the humans. During a skirmish between "Blade" and the Director, the Director was shot and killed. (TV: The Faceless Ones)
Shortly after the Chameleons left Earth, they passed the planet Krennos and decided to kidnap and impersonate its humanoid visitors. After taking their place, the transformed Chameleons would return to their victims' homes for a while to avert suspicion from Krennos, then return to the Chameleons' home world. The Sixth Doctor discovered their plan and put a stop to it. (PROSE: Face Value)
The process used to assume the identities of humans involved the use of armband-like devices placed on both subjects. If an armband was removed from the original, the Chameleon that assumed their identity would be killed, reduced to a small blob of protoplasm. The abducted humans were miniaturised and placed in suspended animation. (TV: The Faceless Ones)