Bernard (The Secret in Vault 13)

Bernard was the name given by the Thirteenth Doctor to a Mark VI combat mechanical who was sent by the Fleet to hunt her, Graham, Ryan and Yaz.

It pursued them through the Forest of Desolation, across the Burning Grounds and into the Valley of Agonies.

After surrendering to it, the Doctor reprogrammed it using a virus that spread to all other Mark VI's, turning them into a peacekeeping force to step the aggression of the Fleet. (PROSE: The Secret in Vault 13)