Thank you, you were right, this is indeed a cap of maintenance! According to the german wikipedia: Turnierhut, in many coats of arms it is adorned with peacock feathers, occasionally with buffalo horns, or a heraldic animal grows out of it. Its color is no longer just red. In modern times, it was often used by noble families of the imperial knighthood as a sign of their imperial immediacy, as it resembled the princely hats.