Hard crusts or mats in a rabbit’s fur are often the result of the rabbit not being groomed often or thoroughly enough; or having bedding or litter that is not cleaned and replaced often enough. Long-haired rabbits require daily grooming of at least 10 to 20 minutes to maintain a healthy coat. However, if you are matching this and you also keep your rabbit’s quarters nice and clean, I suggest a visit to your vet, because there are rare autoimmune diseases that can affect rabbits’ skin, producing crusty lesions in the coat.