The pet dog
Man's best friend represents a person or energy which will come to the dreamer's aid when they need it most. For example, if you were having a dream you were drowning and a dog saved your life, it would point to help arriving at a time of need. Maybe you are feeling depressed and have just given yourself a good talking to and picked yourself up, or maybe someone else has helped you to see things in a new light. The dog symbolises emotional support and comfort in the face of danger, or psychological problems.
The nasty dog
The dog also represents our own, instinctual selves. An abandoned or mistreated dog might show a negative attitude towards your instincts - maybe you are not listening to your inner voice, or neglecting your own needs, like a puppy who doesn't know what he wants. Start listening to both your body and your mind.
The unhappy dog
Mythical dogs are often guard-dog images. A fierce Cerberus, guarding the entrance to a cave, may represent a challenge to be taken before you can pass the threshold to another stage in your life. The sacred dog is linked to personal development, there to help you overcome obstacles and motivate you to move forwards. He's also a ferryman figure, there to help you to recognise and draw on your own resources to get through difficult times, a reminder that you can be strong.
The aggressive dog
A dog which snarls or bites in a dream represents a repressed instinct or feeling, which is fighting its way out into the open. Maybe you are being friendly towards somebody to keep the peace, when really you don't get on, so you are supressing your real feelings and creating tension in yourself. This interpretation can also be applied to other people, maybe you suspect someone of being two-faced or superficial, or think that they have been bad-mouthing you. Be on your guard.