The Zaratan existed in the mythologies of medieval Europe and Islam. Chaosium first used it in 2009 for its Call of Cthulhu RPG.
Zaratan are an ancient race which lives throughout Earth's oceans and seas, and sometimes in it's largest rivers. The race has been around for millions of years, perhaps billions of years. Drifting in the water as they sleep, and due to their size the water's current barely moves them.
Over the years, plant life grows on their shells and birds nest on them. Though its true nature is usually known to those whom live close to it and therefore avoid it, unaware sailors may land on a Zaratan. If they dig too deeply or start a fire, they cause it pain. The enraged behemoth then submerges, dragging the sailors deep into the water in the undertow.