The Praetor was the title of the consul who was the supreme commander in the army. Originally, the Praetorian Guard was a casual term used to describe a collection of elite cohorts, whose function was to guard their commander. Units such as these appeared in the late Republic. After defeating Anthony, the new (and first) emperor, Augustus Caesar, formalized the Praetorian units. There were probably nine cohorts comprised of around 500 men each, just as with the regular legions. Before 2 BC each individual cohort was lead by a tribune of equestrian rank. Afterwards, Augustus created two posts for overall command of the guard. These 2 men in charge were called Praetorian Prefects. The Emperor Tiburius further consolidated the Praetorian Guard by constructing the great Praetorian Camp in Rome. This act so impressed the Guard, that the scorpion became a common symbol on Praetorian shields, armor, etc. (Tiberius' birthsign was scorpio). The primary role of the Praetorians was to act as a bodyguard to the emperor and serve as a police force in the city. However, they did take to the battlefield when the need arose.
Common missconceptions about the Praetorians
1. When illustrated Praetorian Guardsmen are often depicted with blue shields. Yet, there is no direct evidence to for this. Shield blazons on a relief housed in the Louvre are thought to be Praetorian. They include moon and star shapes, which has lead many artists to assume the shield was painted black or blue (to represent the night sky).
2. In film and toys Praetorians often appear in black costume. There is nothing to support this color choice. It is merely an artistic convention distinguish them from the legionaries (and make them look cool!). Tacitus’ description of the 69 AD battle of Cremona implies that Praetorian shield was all that distinguished his appearance from a legionary. (Tacitus, Histories)
1. The Praetorian Guard, by Boris Rankov