Last night I dreamt my ex-girlfriend's dog bit my current girlfriend(?). What does this mean?
Just a small scratch on the shin from a fluffy little dog if that matters.
I immediately thought your dream meant you wanted to have a 3-some and have the dog video tape it. However, after some research, I may be wrong. Though this doesn't exactly describe your dream sequence, it's more about what the "dog" symbolizes in dreams.
Dog
To see a dog in your dream, symbolizes intuition, loyalty, generosity, protection, and fidelity. The dream suggests that your strong values and good intentions will enable you to go forward in the world and bring you success. The dream dog may also represent someone in your life who exhibits these qualities. Alternatively, to see a dog in your dream, indicates a skill that you may have ignored or forgotten. If the dog is vicious and/or growling, then it indicates some inner conflict within yourself. It may also indicate betrayal and untrustworthiness. If the dog is dead or dying, then it means a loss of a good friend or a deterioration of your instincts.
To dream that a dog bites you on the leg, suggests that you have lost your ability to balance aspects of your life. You may be hesitant in approaching a new situation or have no desire to move forward with your goals. Alternatively, it symbolizes disloyalty.
To see a happily barking dog in your dream, symbolizes pleasures and social activity. You are being accepted into some circle. If the dog is barking ferociously, then it represents your habit of making demands on people and controlling situations around you. Are you "barking" too many orders? Alternatively, it could also indicate unfriendly companions.
To dream that you are buying a dog, indicates your tendency to buy your friends or buy compliments and favors. Alternatively, it suggest a need for you to find companionship. If you are being guided by a dog, then it suggests that you are having difficulties in navigating out of a situation or problem.
To dream that you give or send your dog away, indicates that the decisions and choices you are making now may be misinterpreted as disloyalty. You have to do what is right for you and not worry about what others think of the decision. Alternatively, it signifies rejection of friendship.
To dream that you are dressing up your dog, signifies your attempts to cover up your own character flaws and habits.
To see a black colored dog in your dream, symbolizes the shadow aspect of a friend. The dark side of someone close to you is being revealed and you are able to see through to their true intentions.
Also consider the notions associated with the word dog, such as loyalty ("man's best friend") and to be "treated like a dog".