The DSM says:

Diagnostic Criteria

1. A pervasive pattern of social and interpersonal deficits marked by acute discomfort with, and reduced capacity for, close relationships as well as by cognitive or perceptual distortions and eccentricities of behavior, beginning by early adulthood and present in a variety of contexts, as indicated by five (or more) of the following:
   1. ideas of reference (excluding delusions of reference)
   2. odd beliefs or magical thinking that influences behavior and is inconsistent with subcultural norms (e.g., superstitiousness, belief in clairvoyance, telepathy, or "sixth sense"; in children and adolescents, bizarre fantasies or preoccupations)
   3. unusual perceptual experiences, including bodily illusions
   4. odd thinking and speech (e.g., vague, circumstantial, metaphorical, overelaborate, or stereotyped)
   5. suspiciousness or paranoid ideation
   6. inappropriate or constricted affect
   7. behavior or appearance that is odd, eccentric, or peculiar
   8. lack of close friends or confidants other than first-degree relatives
   9. excessive social anxiety that does not diminish with familiarity and tends to be associated with paranoid fears rather than negative judgments about self.

2. Does not occur exclusively during the course of Schizophrenia, a Mood Disorder With Psychotic Features, another Psychotic Disorder, or a Pervasive Developmental Disorder.
3. The disturbance is not due to the direct physiological effects of a substance (e.g., a drug of abuse, a medication) or a general medical condition.

Note: If criteria are met prior to the onset of Schizophrenia, add "Premorbid," e.g., "Schizotypal Personality Disorder (Premorbid)."

The DSM is copyrighted, copies of the diagnostic criteria can be found online in numerous places. This single entry is reproduced here under 'fair use' for educational purposes. Don't edit this section of the text

Many of the criteria could be used to describe otherkin, either in general, or specific people. As such it should be seriously considered as a possible diagnosis. It is also possible that otherkin who are otherwise sane are misdiagnosed as Schizotypal.

The first section is the relevant one. However the first paragraph excludes a good number of people claiming the otherkin label. Otherkin are not (necessarily) geeks with no social life or skills. Many can, and do, have close personal relationships and function well in the societies in which they find themselves.

Ideas of reference

What happens when someone interprets a message specifically for them from something objectively generic or neutral in meaning. Example: A schizotypal may hear a lyric in a song and interpret it as specifically being a message to them from someone else.

Odd beliefs and magical thinking

Whilst this definitely applies to otherkin (seriously, if believing you aren't human isn't an odd belief from a human perspective, I'm not sure what is), it may not necessarily qualify as "inconsistant with subcultural norms". There are subcultures for which magical thinking is normal, the belief in reincarnation is normal, the belief in non-human intelligence is normal. (examples?) Magical thinking, on the other hand, also includes attributing "magical" causes to normal phenomenon, such as thinking that an itchy rash is one's inner dragon trying to shed.

Bodily illusions

Check. The concept of {{True Form}} would not be so pervasive if so many otherkin didn't experience some form of {{Phantom Limb Syndrome}} or similar effect.

odd thinking and speech

This includes things like speaking in an archaic manner, as some otherkin do, using thee, thou, or as the above author had wrote "Whilst..." ;) -- and might also include accents and the like. Anyone want to take a stab at the meaning of "odd thinking" in the DSM?

suspiciousness or paranoid ideation

:This seems fairly common amongst newly awakening otherkin (behaviour patterns article reference?). Most seem to grow out of it however.

inappropriate or constricted affect

Emotional oddnesses. Strange expression of feelings. Feelings are inappropriate to a situaton. Schizotypal is on the Schizophrenic spectrum: it's not uncommon for schizotypals to smile at sad news, to laugh in the face of someone who just fell down, etc. I'm not sure this applies to most Otherkin, who seem to be generically appropriate in feeling/emotional affect. Affect is one's display of emotions; unless you tell a psychologist what you feel, they can only observe outward signs of emotions.

behavior or appearance that is odd, eccentric, or peculiar

Actually you probably couldn't spot most otherkin in a crowd unless you knew exactly what you were looking for. However, get people in their own space, and they will do some things many folks consider odd. (Then again, so do the SCA, LARPs?, college fraternities, sports team locker rooms, etc. when they're in a space they feel comfortable in).

lack of close friends or confidants other than first-degree relatives

If anything, otherkin seem more likely to be estranged from their blood relatives than average, but probably not by much. It's hard to say how much of the difficulty otherkin have in social situations stems from having been estranged or outcast most of their lives rather than vice-versa. Once within their own group, Otherkin seem to be doing ok by "normal" socialization standards.

excessive social anxiety that does not diminish with familiarity

Humans are weird, but they aren't necessarily scary.

So we have two or three definites, when the diagnostic criteria call for at least five. I don't think it fits otherkin in general, but I do think it fits some people claiming the label and should be considered as a possibility on an individual basis.

Conversely, it's possible that many people who are Otherkin have fallen into this diagnosis whether or not they deserved to or would be "curable" by the normal means used for schizotypal personality disorder. It's possible to look at this both ways: Maybe most Otherkin are schizotypal and would benefit from psychiatric treatment. Maybe some schizotypals who respond poorly to psychiatric treatment would have benefitted from being labeled Otherkin and being immersed with their own kind and bopped with a spongy cluebat.