Terrorkons were giant mutations which lived in the Lake of Mutations on Skaro. They had two heads filled with sharp teeth, long necks and tail, and four legs with clawed feet. As they were very aggressive, the Daleks used them as a natural defence barrier for their city, but required the use of electric barriers to protect themselves from them while storing missiles in the Lake.
One Terrorkon broke through an electric barrier and attacked the Daleks working in the Lake. It then stole a missile and headed through an underground river into a cave deep in the mountains. Finding the missile inedible, it threw it away, which activated it. The Dalek Emperor and the Red Dalek followed the Terrorkon, defused the missile, and observed as the creature was attacked and killed by an giant eel. (COMIC: The Terrorkon Harvest)