Full disclosure: I copypasted myself from a similar thread and edited slightly. We get threads like this a lot. I'm starting to think we need a FAQ in the sidebar.
What exactly is otherkin?
A being/soul/personality/mind (delete as appliccable) inhabiting a humaniform body who identifies as something other than human. Some would say a non-human trapped in a human's body, others cite it as a spiritual beleif (usually reincarnation) or a way of life. Many cite all three. Essentially, otherkin are people who do not think of themselves as human, despite their very human body.
Otherkin is a word applied to any identity outside of "I am human". It's a very broad term (that does not include furries, those are different, though there is a lot of crossover) that can have a different meaning to all fifty otherkin you could ask.
What kind of otherkin can you be?
Basically any kind. I've yet to meet anyone who seriously identifies as an inanimate object (except for the occasional robot), but the truth is that if someone said, with all seriousness, that they identified as - for example - a chainsaw, most of the 'kin community would take them seriously and respect that. Extant terrestrial species (which is to say, "real animals on this planet right now") seem to be very common, as are certain species considered mythological such as unicorns and dragons. However, I have met many other species. To offer some examples, I personally have known
Is Otherkin just a Tumblr fad?
Definitely not. I've considered myself non-human for most of my life, and met others who felt the same in the early 2000s. I'm not sure where the word "otherkin" came from but we were using it by 2006. I've never had a tumblr account - well, I plan to start one soon because I've been told it's a haven for certain things most redditors don't like to talk about. It's possible that's where the "it's just a tumblr fad" idea came from - people learned tumblr functioned as a safe place for unusual identities and lifestyles, and so such people collected there.
Is Otherkin a mental disorder?
It's a personality trait, an identity, a beleif, a lifestyle, or a religion, depending on the individual. "Clinical lycanthropy" is a diagnosis thrown at a few 'kin, but it rarely sticks.
Why do most otherkin seem to be teenagers and young adults?
"Otherkin" is a new word. The older ones probably use different words, or don't label it and just live it. Stalking Cat didn't use the word, but he was as much otherkin as he was furry. The oldest creature I knew who used the word "otherkin" to describe himself was forty-seven. You mainly see the young ones because they're the ones using the word out in the open where people can see them.
How do you know that you're otherkin?
A very commonly asked question by those who think they identify as such. Like I said, otherkin is a word. It's a label. Chosing to use that label is a choice, but deciding that the label fits can take time, introspection, and meditation. It's a facet of identity. An element of personality. Most 'kin just know, in an undefinable way. The same way you (I'm assuming) know you're human, I know I'm kalak. And knowing that means the word "Otherkin" applies to me.
Are otherkin related to any other subcultures?
People tend to lump furries and otherkin together, but there is a real crossover there. The truth is that, like any other element of identity, otherkin can be found anywhere and in any subculture.
Do most otherkin keep their identity secret?
There are still homosexuals who keep their sexuality a secret in countries where it's perfectly legal to get married. As long as humans are ignorant and hate what they don't understand, folk are going to keep secret anything that might result in society casting them out.
Why do so many otherkin seem to be "cool" species? Where are the slugs?
Coincidence, perhaps. 'Kin of basically any species you can concieve exist. Certain species seem more common than others. It's also worth noting that there exist what I call "starter species". There seems to be a shortlist of species that most 'kin gravitate towards when first coming to understand that they don't feel completely human. Dragons, elves, wolves, dogs, cats, and fey seem to be on that list. Usually 'kin branch out, they say "...I don't think I'm a dragon. Maybe more like a crocodile?" and begin to realise that they latched onto dragon because it was a more familiar animal, to them, than a crocodile. Of course, some remain dragons and that's fine.
I've met turkey, goat, sheep, finch, sea cucumber, coral, spider, bat, skunk, camel, and scarab otherkin, if that helps.
Do any otherkin only do this for attention?
I suppose if someone wanted to be oucast from society and recieve endless hate, disbeleif, refusal to acknowledge a basic aspect of their personality, and general distrust from most humans...sure, they could deliberately adopt an otherkin personality. I've never known any who do, though.
Is there any connection between non-human identity and gender identity?
None whatsoever. 'Kin can be cis, trans, queer, or nonbinary. Gender and species identity are as far removed as gender identity and sexuality, probably even further removed. 'Kin do use trans* terms (species dysphoria) sometimes, but there is no intention to draw a direct paralell - it's just a convenient term.