What's in a Title? - the Emperors of the Crystal Empire · 3:04am Feb 8th, 2014
Ponies who have been reading Corruption at Nightfall may have noticed that the rulers of the Crystal Empire are titled "Emperors" rather than "Kings." This is because the Crystal Empire was once a Republic, and the title of "emperor" evolved from one given to their chief magistrate, who was commander of their armies. It is exactly cognate, allowing for linguistic drift, to the title of "Chief Commander" used by the Pegasi of Equestria.
Because for thousands of years the Crystal Empire was far stronger than the nation of the Three Tribes which evolved into Equestria, the term "Emperor" came to have a much more impressive significance than the term "Commander," which is actually just the Old Crystallisch word rendered into Modern Equestrian. The leaders of the great Houses of Equestria styled themselves "Princes" and "Princesses" (from a term meaning "first" with the implication of being "first" among the nobility, and for a very long time, both before and under Discord, Princes and Princesses were decidedly inferior to Emperors.
Then, of course, Celestia and Luna defeated Discord and established their keep in the Everfree Forest. They styled themselves "Princesses," in a deliberate move by Celestia to avoid immediately claiming overlordship over Equestria. They then proceeded, through a combination of diplomacy and the overwhelming application of defensive and ally-aiding force, to make themselves in practice the Empresses of Equestria -- but they never used that title, nor even the lesser one sometimes used of a great Prince or Princess, namely "King" or "Queen." Instead they claimed to be "Ruling Princesses," as distinct from lesser Princes and Princesses, such as the Bluebloods descending from Platinum's line.
Of course, by the last centuries of the Age of Thrones, both in armed and personal might Celestia and Luna were in fact more powerful than any of the minor Princes and Princesses whose Houses they effectively made their vassals. Indeed, as the Crystal Empire weakened, they were more powerful than any Emperors. They however wanted the Empire as an Equestrian ally and buffer against the barbarians of the Far North, so they continued to address the rulers of the Crystal Empire as "Emperors" while the alliance remained.
"King" and "Queen" remained in use as royal titles, but were increasingly confined to the barbarian nations. Thus ironically these terms came to indicate less status than that possessed by Princes or Princesses: a King or Queen might lead a strong host but was not as respectable as a Blueblood of Equestria. (The translation from the alien buzzes of the Changelings of the ruler of a Hive as "Queen" meshes with this pattern, though the obvious analogy here is with the breeding caste of eusocial insects, and the Changeling Hives are basically barbarian hordes from an Equestrian POV).
One of the reasons why Sombra, who is actually an Emperor, is referred to as a "King" is because he himself, in his worship of the principle of might equaling right, often called himself such to emphasize that he was at least as fearsome as any of the barbarians who then threatened the Crystal Empire. Another reason is that the Ruling Princesses of Equestria, who ultimately hated Sombra, wanted to call him "King" to emphasize his barbaric nature. By Twilight's time, of course, those Ruling Princesses are her main source of information about him, and she simply follows their lead (even later, after she's had a chance to go through the entire Great Library and better understands the Imperial customs).
I love head-canon that is thought out this clearly and thoroughly!