CMC Scootaloo 
"[@Thunder_Arch":](/images/2102843#comment_8376317
)
No, it would still be an alternate universe then. By that logic, you would have to see the Walter White pony from "Grannies Gone Wild" as a pony who actually lives in Equestria, too. But we know it's just a reference to another TV show. The chance that a pony who looks like Walter White lives in Equestria is little to non-existent.
Characters from other shows who appear in My Little Pony: Friendship is Magic as ponies aren't part of the script, they're inclusions by the storyboarders or animators to make a joke or to reference something popular for the sake of fanservice. Which was the case when Flash Sentry was suddenly seen with a speaking role in "Three's a Crowd" and if Sunset Shimmer where to be seen, then it would be the same.
Although, for such appearances of Sunset Shimmer and Flash Sentry, there is a different and more complex possibility:
Since EQG and My Little Pony: Friendship is Magic show the audience two different universes, the hypothetical appearance of Sunset Shimmer in the latter would then simply be the Sunset Shimmer of that universe, not EQG's universe. Same goes for Flash Sentry.
There are also two different versions of the Sirens, after all. One version was seen in that one EQG comic that they included in "Fiendship is Magic" and who were sent into the human world from EQG by Starswirl after some kind of magical music competition in ancient Canterlot.
While the versions of the universe that is seen in My Little Pony: Friendship is Magic contacted Stygian, seized control of his village and the ponies living there, were eventually fought by the Pillars of Equestria in Stygian's village (as opposed to Canterlot) and sent away by Starswirl through a magical portal that leads who knows where.
And while every EQG fan celebrated the Season 7 Finale and "Legends of Magic" for supposedly acknowleding EQG's existence, it was, for me, a reason to celebrate that the theory I already had since Season 4 got confirmed by Season 7's finale.
My Little Pony: Friendship is Magic and EQG are entirely separate.
And since they are separate, that means that there are not only two different trios of Sirens (the one that got sent to the human world in one universe and the one that attacked Stygian's village and got banished to an unknown world in the other one), but that there also have to be two different Sunset Shimmers (pony) and two different Flash Sentrys (pony).
With their human counterparts living in yet another, third universe, in which ponies aren't ponies, but humans instead.
So, long story short, if Sunset Shimmer were to be seen in My Little Pony: Friendship is Magic, then it wouldn't be the Sunset Shimmer that you've seen in EQG and that went through the mirror to the human world, it would either be a mere cameo by a storyboarder/animator or it would be the Sunset Shimmer from the universe of My Little Pony: Friendship is Magic.
Scootaloo Fanclub Member
No, it would still be an alternate universe then. By that logic, you would have to see the Walter White pony from "Grannies Gone Wild" as a pony who actually lives in Equestria, too. But we know it's just a reference to another TV show. The chance that a pony who looks like Walter White lives in Equestria is little to non-existent.
Characters from other shows who appear in My Little Pony: Friendship is Magic as ponies aren't part of the script, they're inclusions by the storyboarders or animators to make a joke or to reference something popular for the sake of fanservice. Which was the case when Flash Sentry was suddenly seen with a speaking role in "Three's a Crowd" and if Sunset Shimmer where to be seen, then it would be the same.
Although, for such appearances of Sunset Shimmer and Flash Sentry, there is a different and more complex possibility:
Since EQG and My Little Pony: Friendship is Magic show the audience two different universes, the hypothetical appearance of Sunset Shimmer in the latter would then simply be the Sunset Shimmer of that universe, not EQG's universe. Same goes for Flash Sentry.
There are also two different versions of the Sirens, after all. One version was seen in that one EQG comic that they included in "Fiendship is Magic" and who were sent into the human world from EQG by Starswirl after some kind of magical music competition in ancient Canterlot.
While the versions of the universe that is seen in My Little Pony: Friendship is Magic contacted Stygian, seized control of his village and the ponies living there, were eventually fought by the Pillars of Equestria in Stygian's village (as opposed to Canterlot) and sent away by Starswirl through a magical portal that leads who knows where.
And while every EQG fan celebrated the Season 7 Finale and "Legends of Magic" for supposedly acknowleding EQG's existence, it was, for me, a reason to celebrate that the theory I already had since Season 4 got confirmed by Season 7's finale.
My Little Pony: Friendship is Magic and EQG are entirely separate.
And since they are separate, that means that there are not only two different trios of Sirens (the one that got sent to the human world in one universe and the one that attacked Stygian's village and got banished to an unknown world in the other one), but that there also have to be two different Sunset Shimmers (pony) and two different Flash Sentrys (pony).
With their human counterparts living in yet another, third universe, in which ponies aren't ponies, but humans instead.
So, long story short, if Sunset Shimmer were to be seen in My Little Pony: Friendship is Magic, then it wouldn't be the Sunset Shimmer that you've seen in EQG and that went through the mirror to the human world, it would either be a mere cameo by a storyboarder/animator or it would be the Sunset Shimmer from the universe of My Little Pony: Friendship is Magic.