The Thought-Form was a type of shapeshifting symbiote.
Thought-forms were linked with a host, sharing thoughts with each other. Generally, the thought-form took on an inconspicuous form except when hunting. When hunting, they would take the appearance of translucent material, between gas and liquid, with star-like lights inside. They were also capable of passing through matter.
When Alison Lloyd died in a car-crash in the early 21st century, a thought-form was able to bring her back. Taking the form of Mr Pickles, a small stuffed toy, the thought-form protected Alison. When they moved to SkyPoint apartments, the thought-form began feeding on humans. Besnik Lucca, who had cameras throughout SkyPoint, learned of the thought-form and tried to study it. He was able to make friends with Allison and the thought-form in order to protect himself.
Eventually, Torchwood Three learned of the disappearing people and began investigating SkyPoint. Besnik believed that they were going after him and shut down the building. The thought-form believed it was under attack and started feeding on more people (including two failed attempts on Owen Harper). Meanwhile, Alison went to her friend Besnik for protection. When Torchwood finally confronted Besnik, Alison and the thought-form were there at the time. The thought-form attacked, but Owen was able to kill it by injecting it with his blood. (PROSE: SkyPoint)