Imagine what could give Satan nightmares? Hordak is Skeletor's boogeyman.
There's a reason why the Horde was so hyped up back then. Hordak was Skeletor's teacher/master and the legendary Horde were worse than Skeletor and his Evil Warriors! Not only that, but even Skeletor feared his return. (King Hsss too in MYP) We've seen Skeletor in dire trouble as soon as Hordak returns in so many media...like STAR Comics and record books. I don't remember the Hardcover Golden Book, The Horde.
Skeletor should be the main villain of MOTU, but that doesn't mean there is someone worse out there.