The Yssgaroth were horrible, huge, winged snakes with many eyes, but this may have been an illusion since their appearance caused hallucinations. They were native to another universe, but they wanted to enter the normal universe.
The Yssgaroth and their universe were discovered by the Time Lords in one of Rassilon's early attempts to create a black hole. Instead, he accidentally punched a hole 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 (also known as the First Great Time War). Though they were able to push the Yssgaroth back to their own universe, billions of people were killed and whole star systems were destroyed. The Yssgaroth survived and tried to get back into the normal universe. (PROSE: The Pit)