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I guess the horse maintained the yeast cultures

If you are using whole grains there is already natural yeast on it. Yeast is also pretty much everywhere. Nutrient rich solution left open will ferment. It just might not be a super powered brewing yeast.

Scandinavian brewers had a "magic stick" used to capture and store yeast until the next batch.

Brewing, like baking, was a regular occurrence in a household. It was done in the same place, with the same equipment, as last time. The yeast culture from the previous batch was likely a given ingredient.