I’m not sure what the description says, but I will attempt to explain.
The man depicted here is Shohei Ohtani, professional baseball player. He is a great player, and is high level both pitching and hitting. He is sometimes referred to as a “unicorn”, a player so talented that he feels mythical. This is why Twilight Sparkle is confused, as she wonders where his horn is.
Thanks; the quote does say “Is he a unicorn?”