Slarvian

Slarvians were a species that had a similar appearance to snails. The First Doctor heard tales that the Slarvians enslaved whole planets. They were denied entry to the Galactic Federation.

History
They attempted to conquer Earth in 1979, but they were stopped by the Doctor, Susan, Edward Grainger, Linda Grainger and Guy Oliver. (PROSE: Childhood Living)

In the early 2000s, the Forge harvested a Slarvian from a peace conference. (PROSE: Project: Valhalla)

Other references
The Forge's base in the 22nd century could withstand the combined forces of the Dalek, Slarvian, and Cyber empires. (PROSE: Twilight's End)