A Threckon was a member of a species that was, according to the Seventh Doctor, a "vile creature that feeds on negative emotion." It had blubbery, slimy flesh and writhing tentacles.
The Doctor had trapped a Threckon in a fault line under San Francisco. It escaped after three millenia and took over Ned Leech, an Alcatraz security officer. It used Leech to feed on the anger of the inmates. The Seventh Doctor infiltrated the prison, hoping to provoke the Threckon into revealing itself. The Doctor at first mistakenly believed that prisoner Wai-Chun was the Threckon, but then was attacked by Leech. The Threckon was killed by Brad Travers. (PROSE: Inmate 280)