Drago
The draco (“dragon” or “serpent”, plural dracones) was a military standard of the Roman cavalry. Carried by the draconarius, the draco was the standard of the cohort, as the eagle (aquila) was that of the legion.
The draco may have been introduced to the Roman cavalry by Sarmatian units in the 2nd century.According to Vegetius, in the 4th century a draco was carried by each legionary cohort.
History
The draco Standard was originally developed by the cavalry peoples of the steppes, such as the Sarmatians and the Alans, but also by the Parthians and the Sassanid Persians. It may have been used primarily to determine the wind-direction for the horse archers. Arrian described it as a long sleeve, ‘made by sewing pieces of dyed material together‘. This sleeve/tube hung limp when the rider was at rest, but on the move it flew like a serpent and whistled in the wind. The hollow head, in the form of a toothed dragon, was formed from metal and the wind passing through it would extend the cloth tube tail attached to the neck of the head. The draco was also used by the Dacians (or their allies) and no less than 20 of these are shown on Trajan’s Column. Other sources mention Parthian and Sassanid Persian dracos.
Not all such standards had dragon heads. The one below on the left from Trajan’s Column shows the tubular tail with streamers attached. The head looks more like a dog (with ears) than a dragon. The one beside it has a much more serpentine head, and has scalloped rings attached to the tail. Other standards had no heads at all, just the fabric tube, while some had heads looking like wolves or even fishes. These had protruding ears and fins. The images below show part of the Arch of Galerius with several dracos.

The draco in the Late Roman army
The late 4th c. author Vegetius also mentions the draco as a common standard. He seems to have been confused about the difference between the old standards and the new dragons though, as he wrote that apparently standard- and draco-bearers differ, but also that the standardbearers ‘are now named dragonbearers, that both are present in a cam, but also that each cohort has a draco. if correct it would mean the first overall standard for the cohort (where before none had existed between the legionary aquila and the signa of the centuries). the draco may have foreshadowed the later common practise in the 3rd century to permanently detach cohortes from their parent legions.
The East Roman or ‘Byzantine’ draconarius
Around the mid-6th century, the historian Johannes Lydos mentioned the draconarius (drakonarioi-drakontophoroi) in a list of ranks and functions. Justinian also mentioned a corps of 10 draconarii in his edict of 534, issued to Belisarius in North Africa. The military manual called the Strategikon of the emperor Maurikios (582-602 AD) shows that draconarii were probably still around in the early 7th. century. However, it is not clear whether the draconarius mentioned there was already anachronistic or if he still had a proper military function – there was a scholae draconariorum, a non-military office staff of 10 clercs attached to a civilian praefectus praetorio. After the 6th century, the draconarius disappeared from the Byzantine army.