The Cosmic Beasts are creatures of incalculable size that dwell in the Cosmic Sea. They have been known to swallow worlds. Little else is known about them.
It has been speculated that Yorok is a young Cosmic Beast, which has been growing steadily within the Outer Sea in Tarn over the course of millennia. This has led to the prevailing theory in erudite circles that Cosmic Beasts gestate within certain host worlds, and then break free from their "eggs" upon reaching maturity. This may have been why the Old Terethians were developing interworld travel technology.