A horse with a dun color that has dark stripes, or a color pattern in horses that is grayish-blue or slate-colored.
From Spanish 'grulla,' meaning crane (the bird). Applied to horses because their coloring resembled the crane's blue-gray plumage; possibly influenced by Mexican and southwestern horse-breeding traditions.
Horse colors named after birds aren't uncommon—we have 'dun' and 'bay' too—but 'grulla' is unique because Spanish speakers literally looked at horses and thought 'that color matches a crane's feathers.'
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