@Phantom Rider  
It’s not a forcefield - it’s the area where Nightmare Moon’s magic can’t go.

So, how much air does that small force field hold?

@darknessRising24
 
I don’t see how Anubis was implied to be corrupted by Nightmare and Fiendship 4 shows that was NMM’s first time on the moon. The “Home Sweet Home” bit comes off as more sarcasm than anything.

@darknessRising24  
Also, later in FIENDship is Magic, during a talk with Celestia Nightmare clearly implies that she’s keeping Luna prisoner.  
@Harmozel  

 
The mare in the moon is basically the same thing as the man in the moon - an image visible on the moon. Luna isn’t the mare in the moon, she’s causing the mare in the moon.

@Harmozel  
The same reason Twilight Sparkle “had no friends” at the beginning of the series but then Amending Fences came along and “none” became five: Retcons. Plus, after the first episode they switch to saying “to the Moon”.
 
If Equestria recognizes the Moon as a location and not just an object, then using “in” could work for both meanings. Watch what happens if we pretend Luna was banished to “the United States” instead:
 

 
Of course, that’s just a silly semantic experiment of mine and doesn’t prove anything.
 
Interesting trivia: according to an interview, Heather Nuhfer originally pitched a different story for Nightmare Moon’s FIENDship comic that would have included Luna’s transformation and reveal the true nature of the Nightmare Force. Hasbro rejected it for contradicting the show canon and mandated the story could only take place on the Moon during her banishment.

@darknessRising24  
Wait. Why is she ON the moon not IN the moon. It was said in the first episode to explain the mare IN the moon.

@dynamicv
 
That one quote doesn’t directly mean that she’s not a separate entity.
 
Nightmare Moon was the product of some sort of outside entity preying on Luna’s unhappiness at having her night ignored and possessing her. The cartoon does show Luna having a shocked expression as she begins transforming, indicating at the very least that she did not intend to become Nightmare Moon or did not realize what would happen when she did.  
 
Not to mention the way the black mist appears around Luna as she begins transforming suggests that whatever turned her into Nightmare Moon did not come from Luna herself.
 
And if we take the comics into account, Luna was not the first to be corrupted by this thing.
 
In Fiendship Is Magic #4 it showed her not only imply that she’s been banished to the moon before possessing Luna, but also corrupt the inhabitants of the moon into the Nightmare Forces  
 

 
Then there’s the main series in issue #7 where it kidnaps Rarity to use as it’s new avatar to attack Equestria.  
 

 
And then there’s issue #24 with an implication that even Anubis was under it’s power at some point!  

 
As far as I’m concerned, Nightmare is a separate entity.

And yet still no power in dreams.

@HavelDaddy  

I was always fond of NMM being an alter ego of a bitter Luna. Is this series implying it’s an outside entity that possesses others?

@darknessRising24  
Light resist doesn’t always translate into fire resist.

@I Was Never Here  
She’s the Princess of the SUN, I doubt it would bug her.

Her butt is uncomfortably close to that completley open fireplace