There are no such things as "therians" or "otherkin". There is only a social delusion that is linked to the development of the Ego and Id that came about from a desire to be "different" or "special".
The history of this belief is tied to a role-playing game and the development of the Internet. Back in the early 1990's there were several games from White Wolf, including Vampire: the Masquerade and Werewolf: the Apocalypse. At first these were pen and pencil tabletop games, but soon White Wolf developed live-action versions, called Mind's Eye Theatre, and these games tended to attract people from all walks of life, including those with mental issues and a tenuous grip on reality.
When these people proved that they were too "out there" for even this game, they tended to be shut out by the others, as it is fun to play a werewolf, but not o much fun to play with people who believe (delusionally) that they really are werewolves.
Enter the Internet. In the early days live chat was restricted to servers with IRC chat rooms, such as Dalnet, among myriads of others, and a room could be set up for any interest, from cake decorators to hot air ballooning to scuba diving to, well, people who believed they actually were fictional monsters. You see, in real life a person claiming to be such a thing is easily shown to be lying or delusional because of the physicalities involved that easily show that the person is a human, 100%.
But in a chatroom you can claim to be anything, anything at all, even a werewolf, and no one can "disprove" you. Well, no one that is except for Doctors, Nurses, scientists, biologists, anthropologists, and other people who have the education and common sense to know better. So when the Internet community called these people on their claims, demanding proof, then the fantasy quickly morphed into the claims of "psychic vampirism" and "therianism", because such claims aren't physical in nature and therefore cannot be proven, right? Wrong.
So called and self identifying "therians" claim various explanations, including being an animal spirit trapped in a human body, having an animal spirit tied to their own, once being an animal reborn as a human, and other speculations of fantasy with no evidence to back up or support such claims.
Some even claim to undergo mental and physical transformations, becoming their "spirit" or "totem" animal in reality. Since the original days of this social delusion on the early chat rooms, it has been spread and promoted to younger generations by the older ones who didn't realize that they were simply falling deeper into a delusional thinking pattern.
The actual truth is that these people do not display actual animal behavior. Far from it. They do display learned behavior, from watching these animals in person or on TV, and from reading and learning about them. In a lot of cases their behavior isn't authentic to the animal at all, and features incorrect beliefs and misconceptions about how the animal actually behaves. If they really were "linked" to an actual animal spirit, then their behavior would be authentic. In reality, if these people actually attempted to interact with their chosen animal, they would most likely end up badly hurt, if not mauled or killed. The sad case of Timothy Treadwell is evidence of this.
Therianism, is sadly, simply not real. It is a social affectation among a certain group of disaffected youth, loners, and outcasts that satisfies their need to belong, to be different, or special. The problem is that it can easily become a delusion, and is possibly linked to schizoaffective disorders.
"Clinical lycanthropy is defined as a rare psychiatric syndrome that involves a delusion that the affected person can or has transformed into an animal or that he or she is an animal. Its name is connected to the mythical condition of lycanthropy, a supernatural affliction in which people are said to physically shapeshift into wolves. The terms zoanthropy and therianthropy are also sometimes used for the delusion that one has turned into an animal in general and not specifically a wolf.
Affected individuals report a delusional belief that they are in the process of transforming into an animal or have already transformed into an animal. It has been linked with the altered states of mind that accompany psychosis (the reality-bending mental state that typically involves delusions and hallucinations) with the transformation only seeming to happen in the mind and behavior of the affected person.
A study on lycanthropy from the McLean Hospital reported on a series of cases and proposed some diagnostic criteria by which lycanthropy could be recognised:
A patient reports in a moment of lucidity or looking back that he sometimes feels as an animal or has felt like one.
A patient behaves in a manner that resembles animal behavior, for example crying, grumbling, or creeping.
According to these criteria, either a delusional belief in current or past transformation or behavior that suggests a person thinks of themselves as transformed is considered evidence of clinical lycanthropy. The authors go on to note that, although the condition seems to be an expression of psychosis, there is no specific diagnosis of mental or neurological illness associated with its behavioural consequences.
It also seems that lycanthropy is not specific to an experience of human-to-wolf transformation; a wide variety of creatures have been reported as part of the shapeshifting experience. A review of the medical literature from early 2004 lists over thirty published cases of lycanthropy, only the minority of which have wolf or dog themes. Canines are certainly not uncommon, although the experience of being transformed into a hyena, cat, horse, bird or tiger has been reported on more than one occasion. Transformation into frogs, and even bees, has been reported in some instances. A 1989 case study described how one individual reported a serial transformation, experiencing a change from human, to dog, to horse, and then finally cat, before returning to the reality of human existence after treatment. There are also reports of people who experienced transformation into an animal only listed as “unspecified”.
Clinical lycanthropy is a rare condition and is largely considered to be an idiosyncratic expression of a psychotic episode caused by another condition such as schizophrenia, bipolar disorder or clinical depression.
However, there are suggestions that certain neurological and cultural influences may lead to the expression of the human-animal transformation theme that defines the condition.
- from the Mayo Clinic site.