Who? Who is but the form following the function of what and what I am is a mare in a mask.
Of course, I'm not questioning your powers of observation; I'm merely remarking upon the paradox of asking a masked mare who she is. But on this most auspicious of nights, permit me then, in lieu of the more commonplace sobriquet, to suggest the character of this dramatis persona.
Voilŕ! In view, a humble vaudevillian veteran, cast vicariously as both victim and villain by the vicissitudes of Fate. This visage, no mere veneer of vanity, is a vestige of the vox populi, now vacant, vanished. However, this valorous visitation of a by-gone vexation, stands vivified and has vowed to vanquish these venal and virulent vermin vanguarding vice and vouchsafing the violently vicious and voracious violation of volition.
The only verdict is vengeance; a vendetta, held as a votive, not in vain, for the value and veracity of such shall one day vindicate the vigilant and the virtuous.
It's not even a real issue, though. Use your efforts on something that matters more. Something that effects a whole, not a few. They didn't change her in the DVD release, just in what iTunes has distributed. Using Anon to represent something so small is ridiculous, and insulting.
No one can take her away. She will forever be Derpy Hooves, Storm Queen of Equestria, Duchess of Yoke, Thane of Whiterun, and Baroness of Port Krin on the planet Antallos. (Don't ask. The locals are rather embarrassed, especially since ponies don't normally exist in that reality.)
She will live forever in our hearts. And on YouTube.