The Garm

The Garm was a formidable humanoid of vaguely canine appearance who lived in the Forbidden Zone of Terminus. Unknown to some, he could speak, and his fearsome appearance concealed a harmless nature. He was in charge of the decontamination procedure for Lazar's disease.

Biography
The Vanir used a box which generated sonic waves to control him. He had great strength and the power to ignore energy weapons fired at him. He healed Nyssa by leaving her a decontamination chamber where radiation could cure her of Lazar's disease (a rather chancy procedure).

The Garm helped the Fifth Doctor by using his great strength to prevent Terminus' engines from re-activating and possibly ending the universe. The Doctor then destroyed the box enslaving the Garm, freeing him to work with Nyssa to develop a proper cure. (TV: Terminus)

Behind the scenes

 * Garm is the name of a dog in Norse mythology that guarded the entrance to Hel, the land of the dead.