It is not the physical manifestation of a creature, nor is it a mental condition, but rather more akin to the dream quests that a shaman may take.
It is believed that the spirit of a person projects apart from the body, and takes the shape of one's totem animal/spirit during astral projection. The transformation on the astral plane permits intensely surreal experiences.
A theriomorph is a shapeshifter; a being who can assume an animal as well as a human form. A spiritual theriomorph is someone who at least sees aspects of animals in his or her personality and actions, and those aspects shape who he or she is.
More about shamanism and therianthropy