@HJSDGCE  
Well, that and he’s smirking while Tirek is sneaking up on him. When there is known to be a creature running around stealing magic from unicorns, why would a unicorn be by himself in a dark alley, using magic with his eyes closed, and smirking? If Tirek had been thinking straight he might have realized it was a trap, because it’s kind of obvious.
 
I didn’t guess it was Discord from the appearance; I guessed it from the body language. He’s only got two of Discord’s colors (three if you count the horn, I guess) so the appearance really wasn’t that much of a tipoff (though if you’ve already guessed that it’s Discord, the appearance and the cutie mark are big fat hints that you’re right. :-))

@Prof.NightJack  
He has a blue horn.  
The color scheme fits so well.

@HJSDGCE  

It’s Doctor Whooves with a black mane and tail and a tornado cutie mark.

Come on, I bet we all saw this coming.  
That unicorn resembles too much like Discord that it’s too hard not to notice that.

@Prof.NightJack  
That is the greatest thing.

@Background Pony #0606  
He’s a shapechanger. I would imagine that if he wanted to have a pony form, he could easily have more than one. He needed to be a unicorn because at the time Tirek was preying on unicorns, but if he needed to be a pegasus he might choose a completely different form (for one thing, the tornado cutie mark is incongruent on a unicorn because normally only pegasi work weather, so if he took a pegasus form he’d probably choose some equally incongruent cutie mark, like a banana or something implying he’s an earth pony. :) Also that he’s bananas. Also that he likes bananas on the moon, bitch. :))

@Masem  
Same here.

look like a lot of fanfic versions of ponyfied Discord and his cutiemark were destroyed by that moment

I’m surprised we’ve not gotten the Nigel Thornberry edit yet of this

 
 
 
 

The second picture is Discord exactly one frame before turning to his original face so this is as much as we can see for any drawing purposes(still it is unknown if he have yellow eyes in this form)

Tirek; I presume?

I like his cutey mark it’s a Tornado a slight hit somethings off.