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.
Something fun I do to mess with people - insert other animals when you state the typical “bull shit” or “bat shit” proclamations.
“Ah human shit, I forgot my keys.”
“She’s hippo shit crazy.”
“That a bunch of otter shit.”
Never fails to get weird looks.