The Inugami
The inugami is a mythical Japanese creature that resembles a dog. It is a type of shikigami, spirits used by Japanese diviners. They are extremely powerful spirits who act on behalf of their masters, called inugami-mochi, in delivering vengeance or acting as guardians. They are especially known to cause sickness and disease. Because of the power they possess, however, they do not need their masters and can exist independently of them. They are perfectly capable of turning on their masters, acting on their own impulses, and are known to possess humans.
Creation
The popular belief is that an inugami is created by burying a dog to its neck and surrounding it with food that is just out of its reach. As the dog dies over the course of many days, its master continues to assure that its pain and suffering is nothing compared to their own. When it finally dies of hunger, the surrounding food placates the spirit and makes it obedient to its master. An alternative legend has the dog killed by being buried in the same fashion and then having its head sawed off. Besides guarding its master and enacting vengeance there are many blessing that the inugami brings to its master. The inugami-mochi are highly successful, and the favors they grant are often returned with interest. Because others fear the inugami they command the inugami-mochi are often granted all their requests. It's not just vengeance which awakens the inugami, but also envy. The inugami might act on their master's feelings purely of their own will.
Inugami-mochi have to be careful not to offend the inugami, lest it turn on them, and they are typically shunned by others. Some towns even divided into areas of inugami owning households, and non-inugami owning households. The households with inugami often have a small statue of some sort in front of them, and make offerings. It is difficult for an inugami-mochi to find a spouse, and when arranging a marriage there was commonly a check to see if the other family had one. Marrying into such a family was considered a great misfortune, as once an inugami possesses a family it will not leave. Sometimes it is the house that houses the spirit rather than a person, and it's believed that things such as inugami can be passed down through the women in a family generation after generation.
Possession
When the inugami leaves its original body it starts to rot and wither, and often it's uninhabitable when the dog's spirit returns. In these situations it may take control of its master's body and grow more powerful by doing so. Possession by an inugami cures illnesses and brings good health, but the possessed also begins to behave like a dog. Other signs of possession may be pain in the person's chest, hands, and feet. Strong or unstable emotions increase the chances of a person becoming possessed by an inugami or other spirit.