Here's a news flash: Dogs don't wear shoes. Their feet, exposed to the elements, are vulnerable to picking up foreign matter including thorns. You can remove a thorn from your pooch's foot yourself. Removal of a thorn isn't fun for your pooch, of course, but it can stave off infection and prevent further discomfort down the line. While sometimes thorn removal is best left to the pros, you can handle a simple sliver right at home.
Ask a friend to hold your dog still while you remove the thorn. While this isn't necessarily a requirement, it makes the job easier for you, and you're less likely to hurt your dog if he's held still.
Shine a light directly on the affected area so you can clearly see the thorn and where it enters your dog's skin. A table lamp works best for giving you direct, close illumination.
Sterilize a pair of pointed-tip tweezers by cleaning them with rubbing alcohol. Examine the angle at which the thorn entered your dog's skin, as you want to pull it out at that same angle -- otherwise, you may be unable to extract it, and you'll cause the dog sharp pain if not also tear the flesh. Pinch the portion of the thorn that is sticking out of the skin with your tweezers and gently pull, working it out slowly. If it resists, if it is too deep for you to extract or if it is completely buried under the skin, take your dog to the vet. The thorn may need to be cut out with surgical tools, and your pooch may need a topical anesthetic.
Fill a squeeze bottle with cold water and flush the injury site. If you don't have a squeeze bottle, simply pour cold water over the wound -- water under pressure works best but isn't mandatory. Wipe the injury site with a disposable antiseptic cloth or pour a splash of hydrogen peroxide over it, then flush with water again.
Apply pressure to the injury site for approximately five minutes to control any bleeding. A cold compress may help reduce swelling.
Wrap the injury site in a bandage. Change the bandage every day. For an injury as minute as a thorn, your dog will likely not need the bandage for more than a day or two.