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 hitchers 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, while Mrs Lacy became muscular and scaly with sharp teeth. Although the cat was destroyed after being trapped in a police car which Ace blew up, the Hitcher in Mrs Lacy transferred to Ella Cooper whilst she was defending her daughter, Lizzie Cooper, attracted to her rage at seeing her daughter threatened. The Seventh Doctor planned this, as, if not for Ace's interference, he believed that the Hitcher would be a strong defence for the Manor. (COMIC: Fellow Travellers)
The Doctor later told Ace that the Hitchers should never have appeared on Earth at any time, stating that he suspected that they had been drawn there to confront them by the Mandragora Helix. (COMIC: Distractions)