@shadowfire  
…whoa. That actually makes some sense, and I could see the writers coming up with that.

I’ve also been doing some thinking on the subject and have a hypothesis on why our shadowy friend does not have a mirror counterpart:He does.  
We can clearly tell that the shiloutte in the mirror is King Sombre, A.ka: Lord Growlington, master of standing around menacingly  
Now comes my guess:  
We can see the “inverted” ponies on the right and the “normal” ones on the left. Sombre is in the middle because he’s coming out of the mirror also keep in mind our mysterious friend has no counterpart.  
What if the mirror is Sombre’s?  
We all know Sombre’s mirror lets you see your own nightmares, but what if it doesn’t?  
After Equestria Girls, we gain the knowledge that some mirrors have other parallel universes in them.  
What if Sombre came OUT of his mirror and instead of showing you your nightmares, it shows you what it’s like on the other side?  
The ponies on the right are from Sombre’s world and the ones from the left are the usual cast.  
Now Sombre is from the left, who’s remaining to be his counterpart? The mystery pony.  
What’s an opposite of armor? A suit. What’s the opposite of a crown? A Sombrero.  
tl;dr:  
Sombre is the evil counterpart of our mystery guest, possibly a Latin/Spanish pony; and his return/conflict with his counterpart will become the central theme.

i had to laugh when i saw chyssi on the right side :D

@Dylanius  
Attitude wise - they look the same.

Spike is… the same?
 
 
>MFW they switch places at the end and no one cares of notices

@Background Pony #D41B  
The Former has an upside down tooth and the latter’s still in a Tom Baker-ish regeneration.

Spike and Dr. Whooves don’t seem to have changed much.

@Petedash  
Hardly important, and if the show does have another 5 seasons going, it’s going to be pretty ridiculous to keep using it the same way.

@SRD639  
I’m pretty sure the Anti-Mobius in Sonic, was inspired by the Bizarro universe from Superman, and other past comics. The “alternate bad world/universe” trope has been in comics, for almost as long as comics have been around.
 
Y’know, I also heard the green color for Sonic’s doppleganger Scourge the Hedgehog, might be a possible reference to the famous green Sonic or Ashura gliche in the Sonic 2 video game. So it’s a pretty clever Sonic fan in-joke there.

Twist at the end is that Spike’s mirror double is pretty much their version of Justice Lords’ Batman and just waiting for the right moment to take his “unfriends” down, all while capturing Canon!Rarity’s attention - which, ironically, he rebuffs.

lemme add an important tag we should be in “often forgotten”…

Merge reverted the other way because the “reupload” suffered from tremendous JPEG artefacts and was really weirdly scaled.  
Higher resolution isn’t always the same as better quality, people.

dark luna is best luna

Makes me wonder if there could be a Nightmare Moon in the other universe.

spike has always been half evil

this is not fair…  
just now i notice this is not longer my upload…  
just from where the fuck this freaking anon got bigger covers to make a bigger collage?!

@SRD639  
IDW has done mirror universes of MANY comic series for awhile. For Transformers, one of their successful comic line within the franchise, they’ve done the Shattered Glass universe, where Decepticons wage a battle to defeat the evil forces of…. THE AUTOBOTS!

am I the only one who thinks that all the alicorn princess’ in the mirror universe look like gothic dominatrix’s?

The king sombra in that mirror！

The Mirror Universe and Evil Celestia in this comic cover reminds me of something…
 
 

@HeroInferno  
Take a long look at the Mane…. Do you see it? …It’s King Sombra.

Who’s in the mirror? I MUST KNOW, BY ODIN’S BEARD!!

@Background Pony #6B26  
What about his attitude? His alternate self is supposed to be an Antagonist.

I love how Spike doesn’t change in the least.

@RIG
 
@Millennial Dan
 
…Fuck. Well.. Sono did kind of give me the impression of this being inspired by Archie. I had no idea it was milked so often..