Full-on overanalyzing modus from here on
Well... The flag of Equestria the three tribes raise already shows Luna and Celestia. Now that might be a historical anachronism and the theatre simply showed the current Equestrian flag - or, maybe at that time Celestia and Luna already were important or even mythological figures.
I wonder what happened to the pony tribes' old land... maybe the vacated lands where then settled by the bisons, and that is the territory where Appleosia (or whatever the spelling is) is? After all, ponies seemed to only recently have returned to that region...
Also, we can guess now how Prince Blueblood is royalty - the unicorns used to have an own royal line. Odd how Rainbow Dash is addressed as "sir". But maybe she had a crossdressing role and Commander Hurricane was male.
Now as for historical interpretation:
In the pre-classical era, there once was relatively stable civilization, based on trade between the three tribes. Between the three there were clear hierarchical differences, with Earth Ponies being the least powerful tribe, as they had to pay tribute to anybody. And probably a wasteful and inefficient leadership. I mean, the Earth Ponies look the most ragged of all, and yet their leader is the most colourful of the three?
Then a social collapse happened due to a climate shift. Those things happen. The ensuing chaos destroyed the tribute system, and as the worsening weather removed the pegasi' primary source of power, the Earth Ponies grew in importance and standing. This leveled the playing field, and the three tribes became real competitors.
The climate shift also forced large migration movements. It was during those migration that the power of all three traditional leaderships were weakened, as the common population (represented by the assistents) grew more resentful to them, owing to the harsh conditions and the weakening of traditional legitimacy due to the general chaos.
Thus, the story of the cave is actually a parable of sorts, with each of the six figures (who might not have lived at all) representing old and new order each in every tribe. In the course of the ongoing migrations, the old guard was toppled everywhere, a process that was spurred by a charismatic unicorn leader who hence gets to have a special role in the cave parable. That unicorn leader then craftly used the opportunity to unite the tribes into the new nation of Equestria.
That's how I'd interpret this, anyway
Unbent, Unbowed, Unbroken.
Unrelated ME1 Fanfic