Rhomphaia

[Paullus Scipio]

The description of them of them 'discomfiting' the cavalry "[color=#0000FF]....the Thracians did very creditably against the Theban cavalry, which attacked them first and put up a good defence by adopting the tactics of their country, that is to say by charging out in detachments and then falling back again......

I find this sort of action hard to attribute to swordsmen or men equipped with rhomphaias - the "tactics of their country" were to charge out in detachments, throw javelins, then fall back again. That's not usually a successful tactic against cavalry. I think the Theban cavalry were light cavalry in inferior numbers, and this explains their relative lack of success, although they did inflict casualties on the Thracians and drove them off so they won the engagement.

That's certainly possible, but as you point out, not usually a successful tactic against cavalry, not least because the cavalry, being more mobile, can 'charge out in detachments' better than infantry, literally running rings around them.
The whole passage reads:

"When the news reached the Thebans they hastened to the rescue. Coming upon the Thracians before they had gone far, they took away the spoil/booty and, struck terror into them, and drove them to the Euripus, where the ships which had brought them were moored. Of those who fell, the greater number were slain in the attempt to embark; for they did not know how to swim, and the men on board, seeing what was happening, had anchored their vessels out of bow-shot. In the retreat itself the Thracians made a very fair defence against the Theban cavalry which first attacked them, running out and closing in again, after the manner of their country; and their loss was trifling. But a good many who remained for the sake of plunder were cut off within the city and slain. The whole number who fell was two hundred and fifty, out of thirteen hundred. They killed, however, some of the Thebans and others who came to the rescue, in all about twenty, both horsemen and hoplites. Scirphondas, one of the Theban Boeotarchs, was slain. A large proportion of the Mycalessians perished. Such was the fate of Mycalessus; considering the size of the city, no calamity more deplorable occurred during the war"

What happened seems basically clear.The bulk of the Thracians had begun their withdrawal after massacring the townspeople (Mycalessus was a fair way inland), laden with booty. The Theban cavalry came up first, as one might expect and 'bailed them up' until the rest of the Theban force, which included light troops ( reference to archers, though most must have been local peasants armed with hunting spears/javelins, slingstones and hand-thrown stones) and Hoplites, arrived.They must have heavily outnumbered the Thracians, for they make no attempt to stand. The Thracians carried out a disciplined fighting withdrawal, losing few men, but having to abandon the booty. Those still in the city were hunted down and killed. On arriving at the shore the Athenian ships pulled away out of bowshot and it is here that most of the Thracian casualties occurred, mostly by drowning. In the retreat itself, the Thracians seem to have had the best of the running battle, killing twenty cavalry and Hoplites, including their General. The well protected Hoplites are unlikely to have been killed by javelin fire and this to me suggests that actual close combat took place, as does the death of the commander. As well, the Dii "swordsmen" are likely to have been more inclined to close combat than the average peltast, by virtue of their weapons. Since hoplites would also have swords, perhaps the reason the Thracians had the better of the fighting was the use of 'rhomphaia' to hamstring horses and give them a chance against 'aspis' shielded hoplites.....of course the Thebans might have been beaten purely by javelin fire, but in that case it is curious that no Theban light troops were casualties, if it was just a missile fight, and there would be no hoplite casualties ( as at Sphacteria for example) as they'd have stayed back.

Speculation, of course, but it does suggest the Thracians in this instance pulled off something unusual, since peltasts beating hoplites was a cause for wonder ( e.g. Iphicrates at Lechaion), and that in turn opens up the 'distinct possibility' that they just may have been 'rhomphaia' armed......