Mechanicals were created in the shape of living being, such as a human or a cat. They were self-winding and emitted a faint barely noticeable ticking sound. Some Mechanicals were supplied with a sentience and believed themselves to be a living being. Mechanicals could disguise themselves by wearing clothes covering their whole body including the head, such as medieval armour, or by wearing a face. They could speak and exhibit emotions but the faces they were wearing did not have facial expressions. Some Mechanicals could emit laser beams powered by the clockwork mechanism the same way as clockwork torches light up when the handle is squeezed. Mechanicals were difficult to detect by scanning devices because basic clockwork technology existed on many planets way before the development of space travel. (PROSE: The Clockwise Man)
Melissa Heart brought to London two Mechanicals disguised as medieval knights. She used them to find and kill Vassily. In the process they caused several deaths of humans and Katurians alike. Both were destroyed: one was used to repair a clockwork cat, and the other sacrificed itself to save London.
Mechanicals in the form of cats were used by the AI jailer of the real Vassily to observe him and prevent his escape from his exile on Earth and protect him in case of attack. There were many identical cats with fur resembling the Katurian flag. They could shoot lasers out of their eyes. The Doctor and Melissa Heart reprogrammed one. (PROSE: The Clockwise Man)