Equites singulares Augusti
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.
Although designated in inscriptions as a numerus, it appears to have been structured as a regular milliary (ie. 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 horse, 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). The base constructed at this 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.
The equipment of the equites singulares was the same as for ordinary auxiliary cavalrymen. From the Great Tajanic Frieze incorporated in the Arch of Constantine in Rome, it appears that the emblem of the equites singulares was the scorpion, which was emblazoned on their standards and (fourfold) on their shields.
It appears that after some campaigns, detachments of singulares were left behind in the provinces, to form the core of new regular alae, which retained the prestigious singulares title and crack reputation e.g. the Ala I Flavia singularium stationed in Raetia in mid-2nd century.
In AD 312, after the defeat of the emperor Maxentius at the battle of the Milvian bridge, the regiment was disbanded, along with the rest of the Praetorian Guard, by the emperor Constantine I (ruled 312-37). The unit may already have become redundant, if the scholae, elite cavalry regiments escorting the emperor, had already been established by the emperor Diocletian (ruled 284-305). Alternatively, the scholae may have been founded by Constantine as a direct replacement for the equites singulares.
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