To me, Sandy, a proto-Shoggoth is a creature formed from shoggoth-like material, but with more delicate protoplasm. It is weaker proportionally, and lacks the mutational power of a true Shoggoth. However, because of its greater “liquidity”, it is capable of holding a shape and masquerading as a human, at least when contained within tough (leather? rubber?) clothing, and someone’s peeled skin. A true Shoggoth, with its power, heat, and enzymes, would rapidly break through – but the less fearsome Proto-Shoggoth can carry out the masquerade. This, the Tcho-Tchos find useful, and they apply their notable arcane and biological science towards creating and domesticating these entities.