@Mafioso123  
He is a stallion simply known as the observer, who can be seen watching many historically significant events unfolding.

Who is that in the background?

@Background Pony #5A0B  
Sitting so close to Celestia, the surrogate mother who filled so many emotional needs that her own mother, Twilight Velvet, was unable to fulfill due to her need to be away on diplomatic missions so often, makes her feel safe. As a result, Twilight’s magic decreases her frame in response to the feelings of foal-like security, like how a cat kneeds you like your it’s mother, asking for milk.
 
Amazing how head-canon can present an answer to everything, huh?;)
 
Also accepted as head-canon? Spike’s battle with the giant cockathrice.

Why is Twilight a filly?

Scootaloo sleeping on Celestia is too damn adorable.
 
As “Sleepless in Ponyville” showed: Princess + CMC = Instant adorableness.