The dog or cat has an open skin lesion, wound, or ulcer. Sores on the skin of pets can be caused by a vast array of health and behavior issues. Sores are actually quite common in both cats and dogs and can range from mild to severe. Some of the most common causes of sores in dogs and cats include lick dermatitis, hot spots, bee stings, insect bites, flea bite allergy and frostbite. Other causes include injuries, cancer, mange, contact allergy and tick infestation.