"Mr Pickles" was a thought-form that attached itself to Alison Lloyd.
When Alison was in a car accident, the thought-form brought her back to life. It took the form of a stuffed toy called Mr Pickles which she found in the hospital. It also fed on humans to maintain its form. When Alison and her family moved to SkyPoint, Besnik Lucca saw the thought-form feeding and began tracking it. He tried to make friends with Alison so that the thought-form wouldn't feed on him.
The thought-form also fed on Brian Shaw while Gwen Cooper and Rhys Williams were nearby, leading Torchwood Three to investigate. Besnik Lucca assumed they were after him and locked down the building. The thought-form thought it was under attack and began feeding more often, on Gillian and then her husband. It also tried to feed on Owen Harper, but was repelled by his undead body. Eventually, Alison and the thought-form came to Besnik for protection. Since Torchwood was there at the time, it tried to attack them. Owen killed it by injecting it with his own blood, almost killing Alison at the same time. (PROSE: SkyPoint)