Brainspawn were a grouping of parasitic, worm-like creatures.
Adult Brainspawn were around a metre long with four hooked mandibles around the mouth. They would implant their larvae using these hooked mandibles, whence they would travel to the brain and eventually cause insanity. Brainspawn were slowed by low temperatures and tasted like oysters. (TV: Immortal Sins)
Brainspawn were known on at least 151 worlds, where they were almost universally disapproved of. On one of these worlds they were instead considered a delicacy. Jack Harkness had encountered them on several occasions, including once when he was forced to eat one to be polite.
In 1927, the Trickster's Brigade obtained a species of Brainspawn and brought it to America, planning to infect future American President Franklin Delano Roosevelt and drive him insane, causing him to eventually drop out of World War II and change the timeline. Torchwood learned of this and sent Jack to prevent history from being changed. Jack and his new companion Angelo Colasanto found it in a crate that was to be sent to bite the president. When the Brainspawn lashed out at Angelo, Jack was forced to shoot it in the head, killing it instantly. (TV: Immortal Sins)