Horses and bulls make a bit of sense, big animals make big poos. Calling something a big poo means it is very stinky, a huge waste, etc. Very yucky. But bat poos? Those are teeny tiny, small bats make small poos. What do people have against the poos of bats?
Also, what about larger animals? Whale poos are huge, and yaks, buffalos, etc. all make big poos. Don’t ask me how I know.
I was taught that bat shit (guano) is toxic to humans in ways that can make the person “go crazy” before they die. Similar to mercury poisoning and being a “mad hatter” from the days of mercury lined headwear.