Yssgaroth

The Yssgaroth are horrible, huge, winged snakes with many eyes, but this may be illusion, since their appearance can cause hallucination. They are native to an another universe, but they want to be released into the normal universe.

The Yssgaroths and their universe were discovered by the Time Lords in one of Rassilon's early attempts to create a black hole. Instead of creating a black hole, they accidentally punched a hole through space into Yssgaroth's Universe. The Yssgaroth escaped and caused mass destruction. The Time Lords, led by Kopyion Liall a Mahajetsu, tried to stop them, leading to the thousand year Eternal War. Though they were able to push the Yssgaroths back to their own universe, billions of people were killed and whole star systems were destroyed. Even then, the Yssgaroth survived and tried to get back into the normal universe. (NA: The Pit)