Roman auxilia cavalry

 Prior to the Republican period, Rome depended on their non-citizen allies to provide, train, and equip cavalry known as the Foederati. But, when the Republic transitioned into the Empire, Augustus created a regular Auxilia corps. Although still non-citizens, these troops were now a regular part of the Roman army that were paid and trained by the Roman State. A typical cavalryman of an ala would be paid 20 percent more than a typical citizen legionary. Roman auxilia cavalry were usually heavily armored in mail and armed with a short lance, javelins, the spatha long sword, and sometimes bows for specialist horse archer units. These men primarily served as medium missile cavalry for flanking, scouting, skirmish, and pursuit.  Riders and horses were housed together in the same barracks.

Structurally, a cavalry alae of the type ala quingenaria consisted of 480 horseman (ideally) divided into 16 turnmae of 30  men each under a decurio. The ala milliaria, however, contained 1008 horseman divided into 24 turmae of 42 men each.  The overall commander of either type was always a praefectus alae of equestrian rank.

Although commanders of auxilia infantry were appointed by the governors of the provinces in which they served, praefectus alae were appointed directly by the emperor in Rome.  In the middle of the second century another rank was added, the prefectus alae milliariae.  This rank, seldom awarded, was the highest rank an equestrian could attain. It paved the way for a procuratorship, managing the financial administration of an entire province.

From the late 1st century CE onward, a new type of non-citizen force arose in the provinces, the numeri. Numeri, consisting of 100-200 guard and reconnaissance units, were stationed along fixed lengths of frontier and were not sent outside their assigned province on campaign.  Mounted units of numeri were referred to as exploratio and were used to explore and secure areas beyond the frontiers, particularly in remote forest regions such as the Dacian limes.

Technical Reference: Army of the Roman Emperors: Archaeology and History by Thomas Fischer © 2019.

Roman parade helmet from the vicus of Theilenhofen, Bavaria 150-200 CE.  It bears the names of its owners and their troop units: (Turma) PATERCLIANA ATTONIS; behind the left ear protection: (Turma) ATAVLVANI FL(avi) FLAVIANI; on the right side IVLIA ALIQAN; on the forehead: ALIQANDI COHOR(tis) III BRACARAV(gustanorum Turma) NONI.  Image courtesy of Wikimedia Commons contributor Pirkheimer. 

Image:  Much less ornate Type "B" Roman auxiliary cavalry helmet, 1st century CE, at the British Museum courtesy of Wikimedia Commons contributor Michel wal.