Dogs have been around humans for thousands of years so it is not surprising that they are a common symbol of things, both spiritual and mundane.  Generally, they symbolize fidelity, nobility, faithfulness and watchfulness.  They sometimes represent messengers between the material and spiritual worlds.  They can symbolize priests because priests are watchdogs protecting us from the devil.  They are sometimes guardians of treasures or gates such as Cerberus, the mythical three-headed dog that guards the gates to Hell, or, more accurately, the gate to the afterlife.  In ancient Egypt and Sumeria, the dog was a solar symbol and was an attribute of Anubis and Hermes.  Like Anubis, Xoltl, the Aztec god of the setting sun, is usually depicted with the head of a dog.   A dog often accompanies the Good Shepard and healers.  Both dogs and snakes were found in the ancient healing centers of Asclepius.  In witchcraft and sorcery, a black dog often represents evil powers or death, as with the Grim in Harry Potter.  In Buddhism, a lion-like dog is often a guardian of temples and holy places.  That is why, in China and Tibet, breeds were developed that resemble lions.  
    In dreams, dogs can be guides or symbols of friendship and goodwill.  A dog biting you in a dream may indicate you are going to be double-crossed by someone close to you or a business associate you trust.