Tail that wags the dog
This term describes a shift in power, it is also used in relation to the media spinning the news. It originates from Kipling F. Scott Fitzgerald and Von Arnum.

Working like a dog
This probably originates from sheep dogs who work from dawn til dusk and all they ask for in return is somewhere to sleep, food and affection. This phrase describes someone who works very hard or works long hours.

Dogsbody
This is often applied to someone who does lowly work. Apparently it's a Naval term that refers to the food that the junior officers ate in the 1800s.

Dog's dinner
This is used when someone is in such a state they're 'dressed up like a dog's dinner'. Its origins are unknown.

In the dog house
This one is obvious. It's used when a couple have had an argument and one of them has done something wrong. They are metaphorically sleeping outside 'in the dog house'. It is believed that it originates from the kennel dogs sleep in.