So, a question I have wanted to ask here today is, Why didn't Nightmare Moon claim a title higher than princess?

Now before season 5's finale, she had not been given the title princess, but I am sure many assumed that her title was princess right off the bat, because she's just Luna's darker side.

But, think about it this way, what truly kept Nightmare Moon in the alternate present Twilight encountered with her, from claiming the title Queen or Empress?

I mean, for example, we have Queen Chrysalis, and King Sombra, and even the Storm King from the movie, so what force stopped Nightmare Moon from claiming a title above Princess?

Now, I can't say that all villains claim the title of King or Queen.

Discord and Tirek were lords.

Tempest Shadow/Fizzlepop Berrytwist was a commander.

Starlight Glimmer was a dictatorish town leader, which I think is the equivelant of a mayor.

The Pony of Shadows was Darkness incarnate.

I think that's all major villains. Well Sunset Shimmer may have gone for Princess title, but that's a reality that is most likely not gonna be explored.

I can't say that Thorax is King of the Changelings, but he is their ruler and his brother is second in command, and they could be taking the title of princes for all I know.

And Daybreaker's kind of a wild card, since she was only a dream figure.

But this strays from the heart of the topic.

Why do you think Nightmare Moon didn't claim a higher title than princess in the alternate present where she won?

To be honest, I have no idea, I would say possible to make the transition between her and Celestia less uncomfortable but we all know Nightmare is...
ah... *waves claw around side of head* a bit nuts.

Equestria is a principality rather than a kingdom or empire, and thus, is ruled by a princess rather than a king or emperor. Though of course, given its size, it would probably have been considered an empire historically, but using the term "Princess" might make them seem less threatening. Or just be a holdover from when Equestria was much smaller.

Prince is a term derived from the Latin term "princeps", meaning "first citizen"; in a principality, "Prince" is the highest title, and it doesn't mean the heir to the throne, it means the ruler. One modern-day example of this would be Monaco, which has a Prince, not a King.

Queens are seen as evil, and insane people rarely believe that they are evil

I'm inclined to think they originally took the titles of princess since there can be more than one princess, but queen is a singular position.

When Nightmare took control, she couldn't entirely erase Luna, so the part of Luna that still loved her sister had some influence. So Princess Nightmare Moon was Luna's act of rebellion against Nightmare.

I'd say it's becaise the foals writing the damn thing have no respect for her.

I always assumed it was something similar to the Daedra and Aedra. While they have reign over their respective spheres they do not have absolute control over them and it's more of an interest in maintaining those spheres than mandatory governance. For Luna/Nightmare Moon, it's the night and dream realm (along with whatever else she has particular interest in. Princess of Games, anyone?)

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As others pointed out, a princess isn't a strictly inferior title to a queen. Within Equestrian culture and thinking, it is the highest rank. No more than President is in any way inferior to king or queen in our own world. The fact that a monarchy places king/queen over prince/princess is simply how things are named in those countries.

Another example of this sort of quirk of naming is in military ranks. A captain in the Navy (and Coast Guard) means roughly the same as a colonel in the Army (and Air Force and the Marines): an O6. An O3 in the Army is a captain and is equivalent to a Navy lieutenant. To add even more confusion to that, a Navy O1 is an ensign, an O2 is a lieutenant junior grade. Meanwhile, over in the Army, an O1 is a 2nd lieutenant and an O2 is a first lieutenant. So if a pair of people have a dispute over who outranks who, and one says he or she is a lieutenant and the other a captain, it kind of matters which branch of the military they're each in as they might be the same actual rank (O3) or very different ranks (O6 and O1).

Having something like, "Captain of the Royal Guard" throws yet another use of the title in, without a clear indication of just how high of a rank that actually is. Presumably, that is the top rank of Equestria's military, making it equivalent to the highest ranks of general (Army) or admiral (Navy) in our own military (O10, though how many ranks Equestria's military has would give them not necessarily matching up paygrades). Some fics like to express a dual military structure in Equestria after Luna's return, with a Captain of the Day Guard and a Captain of the Night Guard.

When you start looking into ranks of nobility and royalty, you see a lot of agreement in Europe, which lends a lot of weight towards thinking of these titles as being universal, thus creating the sort of confusion you're having. But an Emperor is not a higher title than a King. The King of England, for example, was the head of an empire. Internationally, he was as important as England was, not less important than a neighboring Emperor simply for not selecting a "higher" title.

So, back to Nightmare Moon, she selected/retained the title of princess, because that is what Equestria knows to be their highest title. I'm sure that when Equestrian diplomats, government workers, and so on have conversations about other nations title structures, they have to learn and probably form opinions on the quirk that they use prince/princess in a lesser fashion (among those that do).

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Princess Celestia herself said Thorax was the new king/ruler, just sayin'

Edit: Nevermind, I thought somepony had referred to him as King at least once, but for some reason none of the examples I thought did. I'll keep looking but I'll still say that taking Chrysalis' position would defacto make Thorax king.

Maybe because titles are just another mechanism of the workings of society.
And IMHO by that point Nightmare Moon had already gone past caring about society or what other ponies think.
Her plan to force them to like her and her night failed, so she settled in for the next best thing. They obey her, and that's enough.

Because Princess is more appealing to kids, specially little girls, rather than Queen because in children media Queens are usually the villain and princesses are the heroes.

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That's pretty much like saying Rapunzel's mother in Tangled was evil, or Elsa from Frozen was evil, just because they are a queen. And it's like saying Blueblood is a good guy just because he's a prince, not saying he is a bad guy, but you get my point. That would pretty much argue the fact that Nightmare Moon should be a queen becuase she's evil.

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With your inputs, I can see why Nightmare Moon stayed title of Princess, so thank you for pointing out those reasons.