The extensive history of the otherkin community has been shortened by me, to the best of my ability, to be exclusively things such as first instances, coinings, and moments when otherkin and/or the otherkin community was included in notable media, or was otherwise inserted into the mainstream consciousness. If you're interested in much more thorough breakdowns of otherkin history, you may check the 'further reading' section.
Otherkinity has its roots in Paganism. The Elf Queen's Daughters (EQD) were an early group who considered themselves to be elves, beginning in about 1972 (although estimates range from the late 60s to 1974). Arwen and Elenor, a pair of sisters, formed this group after using a Ouija board to channel a spirit, in order to receive answers and guidance from it. The Elf Queen's Daughters were a circle / sisterhood who wrote esoteric letters of elven lore and philosophy, and, according to the Silver Elves (a coven of the EQD), "Were and are experts and Tarot, Astrology, Necromancy and other various forms of Magick."
It can be assumed that the earliest form of otherkin being elves can explain why otherkin began to use the elven star (a 7-pointed star) as a symbol, on top of it playing a role in Pagan iconography.
The Elf Queen's Daughters published letters and articles; mostly in Green Egg, a Pagan magazine; until late 1976 or 1977, when they stopped publishing letters for several years, although they did not disband at this time.
Also in 1977, there was an early reference to people who identify as incarnate extraterrestrials ('alienkin'). In Carla Rueckert's 'A Wanderer's Handbook' (published in 2001), Rueckert writes of being a 'wanderer', in the sense of being from somewhere other than earth, and living a past life on another planet.
According to Leaf McGowan, around 1979, three members of the Elf Queen's Daughters; Arwen, Elenor and Loriel; came to the conclusion that they were not elves, but were hobbits (with hobbits being a fictional species from J.R.R. Tolkien's series.) The term fictionkin did not exist at the time, but this is the earliest reference of someone in what would be the otherkin community describing themselves using something created in modern fiction, rather than myth or folklore.
The Elf Queen's Daughters and the Silver Elves all have their own extensive histories and goings-on. Again, you may look into them as early examples of otherkinity with what I provide in the 'further reading' section.
In 1990, a student at the University of Kentucky, R'ykandor Korra'ti posted to alt.pagan and other newsgroups about a plan to start a listserv for people who identify as elves. Later that year, R'ykandar founded the Elfinkind Digest, a publicly-joinable mailing list. 'Elfinkind' is a synonym of 'elfkind', which, itself, is a word to refer to all elves as a group. Again, later that same year, Elfinkind Digest 16 was the first use of the word 'otherkind' (with a D), to refer to people who identify as something other than human. It was coined to be a term to include all non-elf 'others' cropping up on the list, and was a branching off of the word 'elfinkind'.
This listserve also had the earliest use of the word 'otherkin' (as a variant of 'otherkind'), referring to people who identify as other than human, in Elfinkind Digest 71. During this time, it seems that the words 'otherkin' and 'otherkind' were being used interchangeably, with 'otherkin' being the less common of the two. The person who coined 'otherkin', as well as 'otherkind', is Torin / Darren Stalder, who said on the subject, "From what I can remember, I got tired of typing elf/dragon/orc/ect.-kin and just used otherkin. It was convenience and practicality. ... There wasn't actual intent to form a new word; just shorthand."
Ah, isn't it amazing, the things that come to be simply due to humanity's laziness?
In 1992, the alt.horror.werewolves newsgroup (often abbreviated to AHWW) was created. It was originally intended to be a place for discussing werewolves in horror movies and fiction, but this later (around 1993) became a place where many people came to talk about identifying as an animal. This is a landmark in the therian community. Interestingly, this seems to mean that the therian community is based neither in Paganism nor New Age, unlike the other communities.
In 1993, the alt.fan.dragons newsgroup was created. Much like AHWW, this newsgroup was intended to discuss dragons in works of fiction, but it became a place where many people discussed identifying as a dragon. Its name is sometimes abbreviated to AFD, and it also has some whimsical nicknames, such as Alfandria.
According to Swiftpaw's article, 'Tracing the Origins of the Term 'Therianthropy'', 1994 was when James H. III was the first in AHWW to propose that animal-people call themselves 'therianthropes,' as a more general term than 'lycanthropy'. He said he hadn't seen anybody else use that word for themselves. Between 1994 and 1995, the word also picked up slang; this is where we get the word 'therian' from, although 'thrope was also used. 'Were' was already in use at this time to describe this concept (taken from 'werewolf'), and continued to be popular for a long time.
According to Willow Polson, around 1996 was the earliest surviving recorded usage of the word 'otherkin' on the internet publicly (as accessed in 2003).
In 1996, the alt.lifestyle.furry newsgroup was created, which was for people who identified as 'furries' themselves, including roleplay, animal totems, and spiritual therianthropy, as opposed to alt.fan.furry, which was meant to discuss furries as they appear in media.
In 1998, the website The Gryphon's Guild was created, which was mainly a place for people who like media about gryphons. However, some participants say that they are gryphons, or other mythological beings. Some meant that they were gryphons in spirit, but others were roleplayers, which makes for a confusing affair when trying to figure out which of these things one person is.
Also in 1998, the newsgroup alt.culture.vampyres was created as a spinoff of alt.vampyres, for the discussion of vampires as they are in real life, rather than vampires as they appear in fiction. The FAQ describes, 'people who think they're vampires, people who know they're vampires, vampires,' and, 'vampires who think they're people', in a list of examples of the type of people that the newsgroup is for.
Also-also in 1998, Roy Wilkinson created the UnicornsUnited.com website for people who are unicorns at heart. The site goes along with his book, titled, 'Are You A Unicorn?', which was published earlier that year, for people who were unicorns (or lions) at heart.
Still in 1998, the Usenet newsgroup alt.mythology.mythic-animals.gryphons was created for discussion of gryphons and other mythological animals. According to its FAQ file, many participants roleplayed as gryphons, but others also 'strongly feel that they are gryphons (or dragons or mushrooms or other species) in heart, spirit, mind or body.'
Kinships, published by Angelic Press, ran from 1999 to 2002 as a print magazine by and for otherkin. It found a few relatively 'mainstream' distributors, including Barnes and Nobles.
In 2001, through Otherkin.Net, a webpage called Temple of the Ota-'Kin was created by someone going by Kinjo Ten. He coined the term otakukin, to mean someone who is like otherkin, but specifically with a fictional species or character. Despite the obvious anime and manga influence of the word 'otaku', Kinjo Ten brings up fiction beyond anime and manga in relation to this term, saying, "The initial concept of a supposedly 'fictional' paradigm and/or cosmology having partial or complete basis in an alternative reality is not uncommon among otherkin. Sections of the community accept as reasonable extrapolations of fact Tolkien-esque elves and fae, Pernian dragons, and other phenotypes resembling or derived from allegedly 'fictional' sources."
In 2003, the symbol of therians was designed in the Wereness forums hosted on therianthropy.org, called the theta-delta, and interlaces two Greek letters; the theta, meant to represent the human element and/or the soul, and the triangular delta, to represent the animal element and change.
In 2004, an article titled 'Identifying Your Otherkin Species: Ten Tips for the Terminally Tantalized' was written by Sprite Revenchatte and posted to Otherkin.Net. Among the tips, the seventh one directly says to not be afraid of identifying with race that exists in fiction. The author notes that, as-of the time of writing the article, 'otakukin' had a bad reputation within the otherkin community, but she states her opinion that many myths are merely old pieces of fiction.
Later in 2004, the Livejournal community From Fiction was created, with the community description reading, 'The goal of this community is to provide a forum for those who believe their soul or identity is derived from a fictional medium, such as anime, game, book, or film. Often this means a strong connection to a fictional character, fictional setting or fictional race, and is normally termed "fictionkin" (sometimes otakukin, otakin, mediakin).' This is the first usage of the word 'fictionkin' that I am able to find, but information regarding the coining of the phrase, sadly, seems to be lost to time.
According to Ketrino Ghoe, around 2005, some therians called into a radio show to talk about therianthropy, but, unfortunately, Ketrino didn't name anything specific that could back this claim up. If true, this incident is of note, though.
Also in 2005, in The Battalion (a student newspaper of Texas A&M University), Sonia Moghe wrote an article titled 'Get furry: Aggies release their animalistic nature', with 'Aggie' being slang for students at that university. It talks about a student who describes himself as otherkin; specifically raptorkin.
Again in 2005, artist Robin Ward presented a solo art show called 'Otherkin', which explored the intersectionality of human and animal behavior, which he mentioned as being inspired by the subculture.
The 'otherkin' entry on Wikipedia also included its first reference to fictionkin during 2005, that being, 'However, those who profess more unusual beliefs are likely to meet with controversy. One example of this is the otakukin, or otakin, community, who associate with Japanese media such as anime or video games.' Indeed, this general attitude towards otakukin/fictionkin of skepticism and condescension by the rest of the otherkin community is a running theme through most of the community's existence.
In 2010, a caller on Loveline (a radio show) explains that she is a wolf therian, furry and fursuiter, and discussed identity and sexuality.
Otherkin community blossomed on tumblr in 2011, including, but not limited to, the creation of the blog 'Kin Speak, which people could anonymously post to, which led to discussions of many topics that people would ordinarily be too shy to address.
Still in 2011, on Sweden's channel 5, a TV series titled Outsiders aired an episode on the furry fandom, including at least one furry who also identified as a therian.
On the Fictionkin.com forum in 2016, after months of discussion, the forum came to an agreement around a series of similar symbols, which all involved what could be seen as the letter F in the English alphabet, or as a key surrounded by rings. The Official News Blog for the website created a poll, and asked the greater fictionkin community to help make a final decision. The most popular of the 7 symbols was one of two intersecting rings with the bitting of the key (that looks like an F) between the overlapping rings, and the handle of the key ending just below the rings.
Around 2020, the idea of 'fictionkin for fun' began booming in popularity on Twitter and TikTok. This newer community of fictionkin is often referred to as the 'kin for fun', or 'KFF', community. Many fictionkin and otherkin are frustrated with the 'KFF' community's definition of kin as something to be done 'for fun' now being the face of the community that was previously based off of serious religious beliefs.
Otherkin have been mentioned and/or discussed in many different books, including Willow Polson's 'The Veil's Edge: Exploring the Boundaries of Magic', Oberon Zell-Ravenheart's 'Grimoire for the Apprentice Wizard', Frances Di Layto's 'Through a glass darkly', and more. Books have also been written specifically about, by, and for otherkin, including the previously-mentioned 'Are You A Unicorn? The Mission and Meaning of Unicorns' by Roy Wilkinson, 'A Field Guide to Otherkin' by Lupa, and the previously-mentioned 'A Wanderer's Handbook: An Owner's Manual for ETs and Other Spiritual Outsiders' by Carla Rueckert.
Several real-life get-togethers for otherkin took place over the years, as well; some of which taking place annually. These are called 'gathers'. These include (but are not limited to), Kinvention North, Walking the Thresholds, NCHowl, and Texas Meow.