Engarian

Engarians are a species made entirely of energy and appears as floating balls of light with white eyes and mouth. They usually stay on their home planet in their galaxy, as when they get really stressed out they can explode into giant black holes and suck everything inside.

The Doctor encountered a runaway Engarian outside its home and returned it home under immense difficulty as the Special Positron Undercover Death Squad, charged with hunting down and destroy instabilities in the universe, turned up to destroy it. (NOTVALID: The Runaway)