The time was when truffle hunters used pigs to root up the tasty -- and pricey -- fungi, but a tendency to consume the profits has caused hunters to prefer Phydeaux to Porky. Dogs will eat them too, but are more easily trained to "give."
A truffle-sniffing dog can be of any breed or mixture of breeds, so long as he has a keen nose (truffles grow underground), strong concentration and a good working relationship with his partner. These are the same characteristics that make a good hunting/sporting dog or a search-and-rescue dog, and the same breeds tend to do well as truffles finders -- retrievers (Labs and goldens) and Shepherds (German, Belgian and Malinois). On the other hand, miniature poodles and dachshunds have been successful as truffle dogs. With one exception, it's more the individual than the breed.
In recent times, an ancient and rare breed has become recognized for its exceptional truffle-finding abilities: an Italian "cane" called the Lagotto Romangnolo. This multipurpose, small-to-medium working dog with a dense, curly coat doesn't hunt truffles by instinct -- no dog does, they have to be taught -- but he has the three major requirements for a truffle-sniffer, is easy to train, and is a fine companion and family dog into the bargain. He comes in many colors from white to black, but mostly earth tones and tweedy or patchy effects. He presents two serious challenges: grooming and intelligence.
His coat resembles that of the poodle and the Portuguese water dog, and he doesn't mind getting mucky on the hunt, so he may need frequent baths and deep brushing. He can be shorn or not, according to your preference, but you may find this useful for maintenance purposes.
The Lagotto resembles the Border collie in his need for mental and physical exercise, which you neglect at your peril -- a bored dog can get into serious trouble. One long walk or a couple of short ones daily will keep his body happy, but the sharper the brain, the more it requires stimulation, so play lots of games like "find the hidden toy" and "Where's Mama?"
To train your dog to hunt truffles, start with a piece of cheese, preferably gorgonzola (an Italian blue cheese), whose smell is said to resemble that of a truffle. Play "hide-the-cheese," working gradually toward burying it in the ground. Don't give him the cheese when he finds it; use another treat instead. When he can find the buried cheese, buy a small truffle and let him scent the cheese and the truffle at the same time. Then bury the cheese and the truffle together for him to find. Once he can do this, work on finding just the truffle -- no cheese. When he can do this, you're ready to go for buried treasure. Look where the soil is warm and moist 10 to 14 days after rain, under the right trees (pine, oak, hickory, birch and beech) and where squirrels and chipmunks have been digging. If you and your dog strike truffle gold, have them inspected by an expert to be sure what you have are edible truffles and not some toxic impostor.