The Equestrian Diarchy: (Head Canon Speculaions Included) · 10:37pm Feb 9th, 2015
Since Celestia and Luna are ruling the nation equally, not as King and Queen, or in this case, prince and Princess, that makes it into a Diarchy, as opposed to a Monarchy.
When we learn of Cadence and later Twilight Sparkle as princesses, this doesn't change the overall Government, it is still Celestia and Luna on the Royal Throne.
Cadene merely takes over the governing of the newly rediscovered Crystal Empire. The Empire, despite the Pompous name, is a smaller state within the larger nation, not the other way around as the name would lead us to believe.
When Twilight is ascending, sh is taking on a new state, one that was under the Royal crown, up untill Twilight Sparkle had been deamed qualified, partially a matter of her age, one may presume?
If Ponyville had a Mayor before Twilight Sparkle arrived, this doesn't change when she ascend to take her throne. The Mayor is merely managing representative and ceremonial tasks. We have a Mayor in every comunity of a certain size, or above, just as we have outside of Equestria.
Duke andDuchesse of Maretonia, where ever this is? Yet, this could be seen as another State within the Equestrian Nation, unless it is stated otherwise later.
We have both Manehattan and Philidelphia, aside from Vanhoofer. Assuming they all have a Mayor respectively. With or without a Prince and or Princess, or any other lower title for the same position in the respective Cities.
Even Appaloosa is bound to have a mayor, unless the Cherif is managing the post for this small community?
