Shamans like don Juan Matus defined their quest as the quest of becoming, in the end, an inorganic being, meaning energy aware of itself, action as a cohesive unit, but without an organism. They called this aspect of their cognition total freedom, a state in which awareness exists, free from the impositions of socialization and syntax. ~ Carlos Castaneda, The Teachings of don Juan: A Yaqui Way of Knowledge
Don Miguel Ruiz calls them parasites, Susan Gregg calls them angels, fairies and gnomes, Ekhardt Tolle calls them “the pain body”, and others call them walk-ins, manifestations of dreams or fears, abstract forces or spirits.
They can be all of the above, except angels, fairies and gnomes which is merely an attempt at fixating the attention on something that cannot be labeled, only experienced. They reside in a realm, according to Castaneda, that exists outside of our own. They are only interested in energy.