Horses, because they used to be everywhere. Step in the street, and you were likely to step in some.

Bull, because it’s very big and smelly and there was usually some around (steer manure is still prized for gardening).

Bats… because it rains from above and is really gross, and you’ve got a high likelihood of getting really sick from it.

Why we don’t go on about pigeon and seagull guano though? I have no idea.