If you look closely, the horses have something stuck to their manes. Those are seeds from the Arctium lappa plant, which stick to fur and clothing in order to spread in a wide area.
I've seen woolly cows completely covered in the stuff, but they don't seem to mind.
Pictures are from the same area as the castle, but from August/September. Fond memories of going for walks there with my grandmother.
Found a picture of cows with seeds on them, but not that many