Hitcher

Hitchers were disembodied entities that were relentless and aggressive. They drifted in the Void and always travelled in pairs. They hitched rides in bodies, but the bodies had to be full of potential aggression, hatred, or rage.

Two hitches latched onto the Doctor's TARDIS and came to Smithwood Manor where they inhabited a cat and Mrs. Lacy. When they wanted, they could reveal themselves: the cat became a humanoid beast reminiscent of a werewolf and Mrs. Lacy became muscular and scaly with sharp teeth. While the cat was destroyed, the Hitcher in Mrs. Lacy transferred to Ella Cooper defending of her daughter, Lizzie Cooper, because of her rage at seeing her daughter threatened. The Seventh Doctor planned this, without Ace interfering, trusting that the Hitcher would be a strong defense or the Manor. (COMIC: Fellow Travellers)

The Doctor later told Ace that the Hitchers should never have appeared on Earth at any time, and were drawn there to confront them by the Mandragora Helix. (COMIC: The Mark of Mandragora)