Prothumos is thumos before it has taken on language, symbol, or concept. It is defined as an "existential warmth" observable in any organism with a sufficiently complex nervous system (in a mammal, even in some birds): an orientation too primitive yet to be called "meaning," underlying the drives to avoid threat, form social bonds, and orient toward the environment.
This concept's function is to show that the theogenic drive is not a leap unique to humans, but the far end of an evolutionary continuity. The human mind inherited this drive; it did not invent it.