Terrorkon

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.

When the Daleks fired their anti-missile rockets towards the warhead stolen by 2K, the launch disturbed a nearby Terrorkon. (COMIC: Impasse)

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)

The Seventh Doctor battled a Terrorkon or a Horrorkon in London's sewers in 2254. (GAME: Dalek Attack)

Trivia

 * The name of the Terrorkons appears to have been inspired by the Terrorcons from 1980s cartoon The Transformers. They were a quintet of Decepticons who transformed into fictional creatures, and could combine to form the combiner robot Abominus.
 * Moreover, the Terrorkons' two-headed dragon-like appearance appears to have been inspired by the Terrorcon commander Hun-Gurrr, whose alternate form was a two-headed dragon.
 * The name may in fact pre-date The Transformers, as The Terrorkon Harvest was published in 1966, twenty years before The Terrorcons made their debut. (COMIC: The Terrorkon Harvest). It is possible (though uncomfirmed) that the Terrorcons were named after the Terrorkons, not the other way round.