From The "BCE" Collection ("Barbarians, Captives & Enemies" on Roman Coins).
Roman Imperial. Constantius II (337-361) AE Maiorina (5.70g). Antioch mint, circa 350 AD.
Obverse: DN CONSTANTIVS PF AVG. Pearl-diademed, draped, and cuirassed bust right.
Reverse:FEL TEMP REPARATIO / Г (left) / ANЄ (exergue). Roman soldier with crested helmet to spearing fallen horseman & holding shield (long oval-shaped with squared edges, bead border and central hub). Sassanid (?) light cavalryman, bearded and wearing head & neck armor, still astride his horse, turns back to face the soldier, extending his arm in a defensive posture, his shield (plain & circular) on the ground to right.
References: RIC (VIII) 132.
Pedigree: Ex-Dr. Busso Peus Nachf. e-Auction 10, Lot 593 (Frankfurt 18 Jan 2020).
Video available of this coin in hand (3rd coin shown, along with two others) & additional photographs.
Notes: Despite the flan flaws & absent patina, an example with valuable detail and good artistic merit, particularly regarding the combat scene. (Interesting Obv. portrait artistry, the eye depicted by a single spiral.)
On many Roman Imperial coins, the captive or soldier's armament, clothing, and beard are key to recognizing his tribe/ethnicity. On balance, they would seem to indicate a Sassanid warrior here, probably light cavalry (or another Eastern/Persian warrior, such as Parthian).
The light armor consists of baggy trousers, a loose-fitting tunic, and cap with neck covering. The helmet/cap may have a central ridge and may or may not be of a single piece with the neck armor (perhaps scale armor or leather/goatskin), which covers only the back and has a slit (i.e., two tails). (Perhaps this is the leather/scale covering meant to be worn under a heavy steel helmet that has fallen off in battle?)
The shield – small and circular, perhaps wicker, perhaps wooden, without decoration – also appears consistent with Sassanid light infantry and (presumably) cavalry. The tunic and trousers, though, are portrayed in baggy, loose-fitting style that are also typical of Roman depictions of Germanic & Gothic clothing. The beard, likewise, fits portrayals of both European and Asian “barbarian” peoples on Roman coins (e.g., Persian, Parthian, Sassanian, Gothic, Gallic, Germanic). The style of neck armor may be determinative.