Hands down the weirdest, and riskiest, type of starter I’m aware of is that which is used to make salt rising bread. Not a yeast-bateria combo at all but a one-bug show featuring a food pathogen called clostridium perfringens, it’s made from potatoes fermented in milk (though even tree bark can be used to start the culture since those critters are everywhere). I’m curious of course, but haven’t gathered the courage to try it yet. I’m told by locals (many of whom had mothers or grandmothers who made it) that it’s delicious though incredibly smelly. Read more about it in the post Salt Rising Bread II.