Slab

Slabs, apart from being concrete blocks used by Humans, were also robotic beings - typically androids - used by various races, typically criminals, who tended to travel in twos. They had a vague form as to not provoke suspicion, but their attitude and internal anatomy were very different - they obeyed the commands of their master, they disrespected other beings, did not speak and had the same composition all the way through. For example, Slabs used by a Plasmavore on Earth (and, due to the Judoon's intervention, on the Moon as well) looked like motorcycle couriers dressed in black, but never delivered anything and were composed entirely of leather, exept for the helmets.

They could be "killed" by sufficient radiation, although the same amount of radiation was ineffective against a Time Lord. (DW: Smith and Jones)