This is Alternate Universe where Twilight born as earth pony. But because Twilight is OP, she is not only an earth pony but also OP earth pony. 
We can say that this is a combination of earth pony genes and Twilight’s magical abilities manifested in earth pony magic.
 
Further background to this artwork. I was reading some fanfic recently about Twilight born as a earth pony.  
It hit me that in these fanfics Twilight’s earth pony life is showed like something bad.  
She newer become Celestia’s student, being earth is bad and in general suffering.  
But. This is Twilight. Motherbucking OP Twilight Sparkle. I will not choose this path.