The First Cohort of the Ninth Legion was a robot Centurion that served in the Iron Legion. He lived in in Roma D, one of the many Known Worlds where the Roman Empire never fell.
He shot and killed a shopkeeper before turning to the Fourth Doctor. He was unable to cope with the impossible calculation of there being two hearts in one human body (not realising that the Doctor was a Time Lord) and suffered a brain seizure. The Doctor wasted no time in removing the robot's head with his sonic screwdriver and identifying him as the First Cohort. (COMIC: Doctor Who and the Iron Legion)