Black walnuts can be typically be found in any holiday nut mixture and are used in cooking and baking all year round. Walnuts in general are not toxic to pets. The concern for these particular nuts is when they become moldy. It's the actual mold spores that form within the walnuts that are the cause of concern. Dogs are susceptible to this type of toxin and are prone to seizure activity. If you have walnuts in your home, please be sure that they stay fresh and out of the way of your furry family members. If you think your dog has eaten moldy walnuts, consult with your veterinarian for care instructions.