Princessipality? · 7:09am Oct 14th, 2016
Today's random thought. I mean, it's not like a Prince has ever been in charge of Equestria, right? So why would it have occurred to anypony to call it a Principality...
(Good job I'm sticking with Diarchy for the Advancedverse, although pre-Luna's Return there were a lot of awfully eccentric ideas floating around about why exactly it was called that, so far as the pony on the street was concerned, starting with a curious form of the pluralis maiestatis and ending with the Princess talking to herself.)
I am, of course, also assuming that whereas in English 'princess' derives from 'prince', in the original Equestrian, it's the other way around: and the Equestrian word we translate as princess derives in turn from very ancient Equish meaning, approximately, "lead mare of lead mares" or "first of the herd of herds".
Depending on which sources you wish to accept as part of canon the answer will differ.
In particular, Journal of the Two Sisters says that Celestia wanted to be called a princess because “it sounds much friendlier.” :) It was because she refused to participate in the internal affairs of any of the tribes and other polities completely, limiting herself, and by extension, Luna, exclusively to defending it and maintaining peace.
My best reconstruction of early history of Equestria based on that and other semi-canonical sources results in a picture where the Alicorn Sisters form a pseudo-state, (which is why every alicorn is automatically a princess) and all the component polities are its protectorates / vassals. Soon after Luna’s Banishment, however, Celestia starts to integrate them together (see Testing, Testing 1-2-3) and eventually the modern Equestria appears to work most like a federation which has “congressponies” (See the comics “Neigh Anything” storyline) with Celestia still handling almost all international relations all by herself and maintaining executive power. As such, it’s neither an *-archy nor a principality, but most probably, “Equestrian Union” officially, and “Equestria” colloquially, and is fairly unique as far as forms of government go.
Here’s a picture of a grain of salt to take it with, though:
c2.staticflickr.com/4/3797/13914873386_1b3e53a66c_b.jpg