The Shan tribe of Thailand has a powerful spirit that permanently lives within the person, transforming them into witch.
The witch has many powers including shape-shifting into animals.
Phi Phu is passed down through family. Its also contagious, if you share too many meals with a phi-phi, you can be infected. Marrying outside the tribe will bring a Phi Phu into the family.
Phi Phi can send human doubles away. As long as a phi-phi obeys social expectations and laws, it will be accepted. The phi phy is unfortuanate, but not malevolent. The only way to exorcise a phi-phi from a person is that person becoming an ordained Buddhist monk.