Board Thread:Lore Discussion/@comment-29559990-20140204211309/@comment-17114085-20140208115830

That wasn't Nocturnal's true form, it was just one of her forms. It never mentionsin the game that that was her true form. Daedric Princes can look like anything they want but chose to look like something that reflects their personality. Like Sheogorath for example, the reason that the Hero of Kvatch(aka New Sheogorath) looks almost the same as the old Sheogorath(aka Jyggalag) is because that form is what reflects his personality the best.

Now about Boethiah, we have never seen any of her forms, only statues. So until we see her in person we won't know which form she prefers.