Here Twilight is dressed up as Rogue, the power absorbing mutant/hero from X-Men after Twilight looked up for someone she could relate to. So mainly she decided on dressing up as Rogue. She at first thought it wasn’t too much of a deal but she slowly got into it.
 
I mean, think about it, she’s got the stripes on her hair like Rogue does for herself. So it’s a perfect match for the student of Princess Celestia, just like Rogue was a student to Professor Xavier from X-Men too. It’s Fits her perfectly.
 
Her powers are of course flying and being able to take any ability from someone by touching them.