The equites singulares Augusti (lit: "personal cavalry of the emperor" i.e. imperial horseguards) were the cavalry arm of the Praetorian Guard during the Principate period of imperial Rome. Based in Rome, they escorted the Roman emperor whenever he left the City on campaign or on tours of the provinces.
The unit's origin is uncertain but it appears that they existed during the reign of Trajan 98-117 CE. The unit is documented on Trajan's Column as active in the Dacian Wars (101-AD). It has been suggested that they were formed of Trajan's personal horseguards during his governorship of Germania Superior.
Although designated in inscriptions as a numerus, it appears to have been structured as a regular milliary (i.e. double-strength) ala of the Auxilia, under the command of a tribunus militum, who probably reported to the Praefectus Praetorio, the commandant of the Guard. Initially, it probably contained 720 horsemen, divided into 24 turmae, or squadrons, of 30 men each. Numbers rose to around 1,000 under Hadrian (r. 117-38) and the regiment was expanded to some 2,000 horse in the early 3rd century by the emperor Septimius Severus (ruled AD 197-211).
Their homebase was in Rome constructed at that time for the expanded unit has been excavated underneath the Basilica of San Giovanni in Laterano, Rome (see Castra Nova equitum singularium).
The equites singulares were recruited from serving cavalrymen in the alae of the Auxilia, selected for their quality. As membership of the Praetorian Guard was strictly limited to persons holding Roman citizenship, it appears that recruits to the imperial horseguards were granted citizenship on enlistment, instead of having to serve 25 years to qualify for citizenship as did their fellow alares.