Related to your question: in the original Japanese version, Jigo says that the soup "tastes like water". When translated literally, this would seem like a pretty mild comment to an American audience, but in the context of Japanese culture this is a much more insulting thing to say. In order to convey Jigo's blunt words, the line for the English dub was changed to "tastes like donkey piss". I can't answer your original question about donkeys, but I wanted to point out that this specific line is the product of cultural translation and shouldn't be taken literally.