The word drungus/droungos/drouggos could mean different things in the sources of various eras. It could mean an irregular order adopted by any unit (including both infantry and cavalry), or it could mean a very specific type of unit, or it could be used as a synonym for names of units, as it was later used for example by the sixth century Strategikon (2.2.1–3), by Leo the Wise (e.g. 4.44) and Sylloge Tacticorum (e.g. 35.2) in the tenth century. However, there seems every reason to suspect that the word could be used in similar manner also in the third and fourth centuries on the basis of Vopiscus’s references to units with this generic term.
Flavius Vopiscus (HA Probus 13.7, 19.2) has two references to the unit called drungus (droungos), which meant an irregularly formed unit – a throng. In the first instance Probus took from the defeated Germans 16,000 tirones (recruits) who were distributed among the numeri and milites of the limites (frontiers) in groups of fifty or sixty men. In this case the numeri can mean the irregularly organized national numeri or any auxiliary unit, but the former is more likely because after the granting of citizenship to the freeborn men by Caracalla the auxiliary units came to consist solely of citizens and could therefore be considered as Roman soldiers. The milites would traditionally be interpreted to mean the legions but considering the time period I would suggest that in this case it means both legions and auxiliary units. The problem with this is that Vopiscus fails to specify whether the recruits consisted of cavalry or infantry, or both. I would suggest both.
In the latter instance Probus paraded in his triumph barbarians (included Germans and Blemmyes) in groups that had up to fifty men each. In this case it is clear that the size of the drungus was not uniform, and that the men walked on foot.
On the basis of the above, I would suggest that the drungus was any irregular throng of men and that the word could be applied equally to any unit that assumed an irregular order or which was irregular in size. In Vopiscus’s vocabulary the drungus meant a small throng of up to fifty men, or alternatively about fifty or sixty men. The word could also be applied to units of foot and horse. It is possible that the fifty-man units were cavalry units (e.g. 10 files 5 ranks) while the sixty-man units were infantry units deployed in the manner specified by Modestus, but this is pure speculation.
Meanings of terms changed later because the sixth century Strategikon (2.2.1–3) already connects the droungoi with the larger units called moirai (1,000–3,000 men) and mere (3,000–6,000/7,000 men) rather than with the smaller units that had at most 50 or 60 men as in Vopiscus’s text. This may mean that the droungos was smaller than the moira with the implication that it could be identified with the tagma/bandon of 200– 400 men, but this is uncertain because the Strategikon fails to state this clearly. The important points in the Strategikon at 2.2.1–3 are that the moirai, mere and droungoi were all expected to be flexible in size and that the droungos was now a larger unit than it had been previously. By the tenth century the drungus/droungos was a synonym for the units called chiliarchia or moira, which were not to possess more than 3,000 men each. However, the word still retained the meaning that the unit was arrayed in irregular order. The droungos/chiliarchia/moira consisted of units variously called tagmata (sing. tagma), banda (sing. bandon), or allagia (sing. allagion) as it was called in the Sylloge Tacticorum. The ca. 6,000-strong divisions (Roman legions or catervae) were then called mere (sing. meros) or tourmai (sing. tourma). The meros consisted of three droungoi/moirai and were not to have more than 6,000 men (Leo) or 9,000 men (Sylloge). Note that the turma/ tourma was no longer the small 30–32 horseman cavalry unit of earlier times. This clearly reflected a tendency to inflate the terms, because the turma was already identified with the 500-man unit of mounted archers in sixth-century Lydus’s De Magistratibus (1.16), which in its turn used much earlier texts as its sources. It is therefore not surprising that the meaning of droungos was similarly inflated. At the time of the writing of the Sylloge (the date is contested, but it was in the tenth century), the infantry tagmata/ banda consisted of 200–400 men, while the cavalry banda/allagia consisted of at least 50 men or at most 350–400 men. This would mean that at that time the 50-man units were identified with cavalry and that they were no longer called droungos. By then the droungos was a unit consisting of 1,000–3,000 men.1
In sum, I would suggest that in practice the Romans were employing smaller units of flexible size and organization called drungi/droungoi because their size varied and they were formed by choosing a suitable number of men from existing units or detachments. The most natural way for this would have been to divide the existing larger units into smaller throngs, for example because the unit was no longer close to its paper strength as a result of illnesses or injuries or for some other reason. Later Roman military treatises such as the sixth century Strategikon and the much later Leo’s Taktika and Sylloge Tacticorum specifically refer to this when they state that the size of combat units varied according to the availability of men. The Strategikon also notes that the depth of the cavalry unit depended on its quality. The better the quality, the shallower the formation; it varied from five to ten men. Even though this instruction is first found in the Strategikon, it is clear that the Romans had always followed this same policy. The depths of the units, including both infantry and cavalry units, had always been varied according to the quality and size of the force.2