In the Neighrutoverse, all Uchiha are Alicorns.
 
Sasuke used to have a white body, but I didn’t like it and thought the colour scheme was better suited to Sai. So now he is a blueish grey and he looks so much better.  
I imagine that all Uchiha get the first part of their cutiemark when they awaken their Sharingan, which is the Uchiha fan. And then when they find their special talent it becomes a part of their cutiemark. Itachi has feathers for his genjutsu powers, and Sasuke has a lightning bolt for his lightning nature chakra.
 
And that black thing on Itachi’s back is supposed to be a crow. A very, ugly crow. Birds are evil.
 

 

 

 

 
 
Naruto (C) Masashi Kishimoto  
My Little Pony (C) Hasbro  
Picture by the one and only, *Sandy–Apples  
Sandy–Apples ~ 2013