i love this guy and idk why but I wanted to draw him as an alicorn. I guess I was just mostly curious as to what he’d look like is all…  
Who knows. Maybe Stygian managed to gain control of the Pony of Shadows or something and absorbed all of his power (and his wings )
 
The long mane is less an alicorn design thing and more because I really liked his long mane in the Legends of Magic Annual 2018 comic–that alternate-world Stygian OMG I REQUIRE MORE OF HIM
 
I like to imagine Stygian’s cutie mark being a spilled ink bottle and quill  
For one reason, spill ink on a piece of paper–or on anything really–and it just… takes over. Like how the PoS took over Stygian; the spilled ink’s sorta symbolic I guess  
Second reason: In A Rockhoof and a Hard place, he’s mentioned to be a best-selling author, so, y’know, ink and quill :3