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.

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.