Alikat
Oh pleeze! I understand the nature of Don Quijote far better than you think.
I read the book in it's original language for the first time when I was 11 years old.
I stand by my previous post on the matter. Furthermore, "horse's ass" is a derogatory term which is widely used in the English language. Castillian, although not devoid of it's own terms of ridicule, does not employ ones from other cultures/languages.
Now, by the same token, of popular usage in Spain for an individual who does absurd and foolish things, is an exclamation of the noun; ¡CHORIZO! = a sausage.
There are many names and types of sausages depending on the meat used and the region.
CUIXOT = Popularly consumed with bread, rubbed tomatoes 'a la catalana' and rubbed garlic. Also nice in tapas. Not good if you keep a strict Kosher diet.
At any rate, I know of no Don Quixotes anywhere at this time who are a "horse's ass".
That's not to say "horses' asses" don't exist.