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. It was 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.
Not all such standards had dragon heads. Some had heads looking like wolves, dogs, snakes or even fishes. Other standards had no heads at all, just the fabric tube.

                                               

The Draco was adopted first into the Roman cavalry during the 2nd century AD, probably during or after the Dacian wars. The Romans first began to use the draco in cavalry games, the so-called 'Hippica Gymnasia'.
Points in the game were scored for strikes on the tail piece of the dracos carried by a team acting as 'targets', from dummy javelins thrown by another team of riders.
              It is not documented when exactly the draco was adopted as a normal standard for all troop types. The Historia Augusta mentions that the mother of Severus (193-211 AD) dreamt of a puple snake before his birth, something very alike what we later hear of the Imperial standard. But since this source was probably compiled later, we can't be sure this has any bearing on a dating. We are on more solid ground with the entry of the reign of Gallienus (253-268 AD), when legionary troops are said to have paraded with a dracon amongst the standards of the legions, and the troops of Aurelianus (270-5 AD) also had draconarii amongst the standard-bearers. This may lead us to conclude that the infantry began using dracos during the late 3rd c. On the Arch of Galerius, which was built before 311 AD to commemorate Galerius' war against Persia in 290 AD, several dracos can be seen carried by infantry as well as cavalry.

The Roman draco developed into a real dragon with scales and a crest. The only fully preserved draco was found in the Limes fortress of Niederbieber in Germany, which dates to the 3rd century.

During the late 4th century AD, the draco became a common standard for the cohort which was a tenth of a legion.

By 357 AD, the Emperors Constantius and Julian had personal dracos sewn from a purple material. Ammianus Marcellinus writes of flags as well as draco being purple and mentions them making a hissing sound in the wind. The Emperor's personal draco standards made them identifiable in the heat of battle and may therefore have been mainly a tactical instrument rather than a personal adornment.

By the fifth century, as may be deduced from inscriptions from Perge and Prusias/Üskübü, Turkey, as well as a poem by Prudentius, there was a rank called magister draconum. This officer was the superior of the draconarii in a unit. However, we don't know if he directed the draconarii in battle, or may just have been the head of the standard bearers' club or scholae.

The military manual called the Strategikon of the emperor Maurikios (582-602 AD) shows that draconarii were probably still around in the 6th. Century. However, it is not clear wheather 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.

Dracos continued in use in the Caucasus and Georgia, while in the West the Franks under Charlemagne may have adopted them again. This may show continuity, or else Charlemagne's attempts to eminate the Roman Army.
We also see it used at the battle of Hastings in 1066 AD, where it is carried by Harold Godwinson's retainer at the moment of his death.
The next image is from a 14th-century manuscript of L'Histoire de Merlin by Robert de Boron. It shows King Arthur in combat, brandishing what can only be a medieval representation of a Late Roman draco.
The last image shows the flag of modern Wales, called Y Draig Goch (The Red Dragon). This red dragon, as the tale goes, would go back to the myth of the red and white dragons fighting on Vortigern's fort at Dinas Emrys in Gwynedd, Wales.- Read more in – The National Flag of Wales..

** Article taken from: www.fectio.org.uk