Gurnian was a swamp planet that once was the location of a Dalek base. The swamps were infested with vicious, two-headed monsters known as horrorkons, which were kept under control by a squad of Marsh Daleks.
In the 25th century, a ship was captured containing the two humans, Andy and Mary Stone. The Daleks stationed on Gurnian decided to cast their prisoners out into the swamps to see if they would be devoured by the horrorkons. Andy and Mary found the horrorkons preferred a meal of metal to flesh. They determined this by the docile behaviour of the creatures toward them and the mutilated wreckage of a Marsh Dalek found nearby.
They used this to their advantage, leading the horrorkons toward their ship with metal scraps. The horrorkons promptly attacked a squad of Daleks investigating the ship, decimating them and allowing Andy and Mary to escape. (COMIC: Monsters of Gurnian)