The Praetorians actually began their existence as the personal bodyguards of Republican generals; the name of this corps was the "cohors praetoria." Augustus took the men from this corps in order to form the modern image of the Praetorian Guard; as a result, Praetorians were initially a general's personal bodyguard, probably drawn from his best legionaries, and were then transformed into a large corps (most estimates stand at 10,000) that was also probably mostly drawn from provincial legions who had seen their fair share of combat.

Source: Sarah Bingham's The Praetorian Guard: A History of Rome's Elite Special Forces