Slarvian

Slarvians were a species from Slarvia 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. (PROSE: Childhood Living)

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 Third Doctor once told an amusing anecdote to Alistair Gordon Lethbridge-Stewart which involved a Slarvian in a Santa suit holding a rubber chicken. (PROSE: Faithful Friends: Part 1)

While living a simulated life of fame and fortune in the Pleasure Prison, Panda told an amusing anecdote involving a Slarvian and a man wearing a Quark suit in a bar. (AUDIO: Whatever Happened to Iris Wildthyme?)

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

Missy had plans to steal the Slarvian's Astral-Core. (PROSE: Lords and Masters)

Behind the Scenes
The Slarvians originated in Mark Wright's audio drama The Slarvian Menace, part of the Big Finish Productions The Tomorrow People series.