Photo courtesy of Creature Concerns, Inc (creatureconcerns.org)
Porcupines are round, slow moving rodents with quills instead of fur, scales or feathers. They are typically shades of brown or grey in color. Porcupine quills protect them from predators, such as your dog and once the quills are stuck into something, they do not come out easily or quickly.
Porcupines weigh 11-36 pounds, are approximately 24-37 inches long with an 8-10 inch tail. They eat clover, bark, leaves, and herbs so they are considered herbivores. The North American porcupine; as photographed here in Alva, Oklahoma, does climb trees in search of food, but they are mostly nocturnal, which means moves around mostly at night. Porcupines can do a lot of damage in the yard when looking for food or kill a tree quickly by eating bark off. Porcupines are attracted to anything salty, including tool handles, clothing, doors, footwear and anything salt came into contact with. Be aware of this when putting down salt in the winter for snow and ice.
If your dog attacks a porcupine, before you start removing porcupine quills, consult your veterinarian and watch this video about porcupine quill removal from a dog.
Dig Defence Animal Control Commercial Grade is a great product to get rid of porcupines and keep them from digging under your foundation, deck or shed.