Demonic pig spirit

Long ago, a mighty demon was banished to Earth. It wandered, disembodied, until it found corporeal form when it possessed the pigs owned by the managers of the Sow in Rut inn. The demon made the managers of the inn butcher their customers and offer them to the pigs.

Decades later, after all the pigs had died, Sarah Jane Smith investigated the story when the spirit hijacked K9 and made him attack her. She was forced to damage K9 to restore him to normality. K9's possession was attributed to Brendan Richards having earlier played a computer game on K9, with a computer virus making K9 take on the identity of the hog monsters from the game. However, this assumption was untrue, as the spirit of the possessed pig lay in the darkness, eyes glowing red, wanting revenge on Sarah for banishing it from K9, its last hope at obtaining a corporeal form... (ST: The Sow in Rut)