And other things.
Sombra was based on Sauron. And Sauron was one of the Ainur. Sombra had a wavy mane like the princesses, had his horn, he roared, had canines, and became a darkness monster somehow.
McCarhthy said he was supposed to be a presence like Sauron. If so, then why a unicorn? Having him be a unicorn into dark magic doesn’t make him all that unique since other unicorns and alicorns can use dark magic and humanizes (or ponyizes in this case) him too much. You could have had him be a darkness creature from the start which would have fit better with what McCarthy intended.