User blog comment:ImperiexSeed/Supernatural Mysteries/@comment-3481504-20130307031256/@comment-3481504-20130307182246

Yes, in Supernatural, they're similar in form to an eel or serpent.

However, it lore, they're basically gigantic alligators.

I always though the Earth came before the Leviathans, but apparently, a lot of users think Heaven came first. Lucifer does call the Earth "God's last great handiwork" - handiwork here meaning a baseline, a supportive base. So, it's noting a place not a being.