Teselecta

A Teselecta, was a machine, staffed by a crew that have been miniaturised by a compression field, that could travel through time, and was used to locate people who were deemed to have broken the law but had escaped punishment. By finding them near the end of their respective timelines, the crew would then administer their own punishment.

The Doctor, Amy Pond, Rory Williams and River Song encountered the Teselecta known as "Justice Department Vehicle 6018" in Berlin, 1938.

A Tesselecta can also adapt itself to resemble anyone or anything whose likeness it has scanned. During it's time in Berlin, 1938, 6018 took on the forms of a Janitor, a Nazi official, a Nazi soldier (both with and without a Motorbike, as well as Amy Pond and River Song. A member of the crew also stated that it had once taken the form of Rasputin, but with a green skin colour. (DW: Let's Kill Hitler)