A Shoggoth is a massive amoeba-like creature with multiple eyes floating on the surface. They were created by the Elder Things to be controlled with hypnotic suggestion, but eventually they developed independent minds. Tom Swyfte and Frank Reade Jr. stumble across a Shoggoth that consumes Reade and drives Swyfte mad. A later expedition by Miskatonic University also ended with the discovery of a Shoggoth, which produced similar results.
The Shoggoth is a being in H.P. Lovecraft's Cthulhu Mythos. It appeared most prominently in his novel At the Mountains of Madness. The LoEG Shoggoth is also influenced by the alien from John Carpenter's The Thing.