Some might have seen that I have already made an explanation of terms in the comments, but I think I need to update it.
Otherkin - someone who identifies as something nonhuman. Usually kintypes are mythical creatures, but not always. Otherkin is often considered to be an involuntary identity. There's also a definiton proposed by @/aestherians: "To be otherkin is to identify as something nonhuman on an inherent or integral level". [source: link]
Fictionkin - someone who identifies as something fictional, usually a fictional character.
Therian - someone who identifies as an animal (no matter if living, extinct, mythical or fictional animal).
Fictionkin and Therian are also commonly considered to be involuntary identities.
Otherhearted - someone who strongly identifies with something that it impacts one's sense of self (usually it's an animal or a fictional character, but not always). Hearttypes can be either voluntary or involuntary.
From Common Themes in the Early Animal-hearted Scene [link]:
-An element of choice or voluntariness- I was surprised by this, honestly, because it seems like this aspect has largely been forgotten. There was a strong theme of one’s animal identity being something that can be brought forward willingly or otherwise ignored. A number of people described it as a choice to represent or acknowledge themselves as an animal. Inducing phantom limbs and animal-like mindsets voluntarily is also a running theme.
Synpath - basically the same as otherhearted but this term seems to be rather used when it comes to fictional characters.
Otherlink - contrary to popular opinion this term is synonymous with copinglink [coping- part came from "copingkin" which was kind of a precursor (I'm not sure if it is the best word for it, though) of both copinglinkers and KFF]. "Otherlink" was coined probably because "coping-" part was giving an impression that copinglinking can be only a coping mechanism.
From official coining post of the term copinglink [link]:
Copinglink, n. (Tumblr comm.; Coined by Who-is-Page/Stormygio) [E. coping referencing to the early use of “copingkin” + E. link create or imply a connection between. Pl. Copinglinks.]
1. A non-human identity (or in some cases, relation) which is consciously created. The creator is able to change and pick who/what they identify as/with as need dictates. This identity can be created by anyone, and thus, is not dictated by one’s mental health.
2. A consciously created coping mechanism which centers around knowingly, willingly, and actively identifying as (or in some cases, with) the user’s choice of a non-human entity.
3. (2017) A (fictitious) human identity which is taken on as consciously created coping mechanism.
Also, and here's also definition of copinglink from the original coiner (@/who-is-page), which li wrote in a reblog to another post [link]:
Copinglink is not synonymous with 'copingkin.' Copinglinks do not require someone to have experienced trauma, and, despite the name, copinglinks do not need to even be formed specifically for the purpose of coping. Copinglinks have also never referred to specifically voluntary identities which center extinct or extant non-fiction animals.
(Sorry for so much talking about copinglinking/otherlinking, but it is a common misinfo that copinglink is only for the coping purpose).
There's also alternative definition proposed by @/aestherians. Definition with some context (I'd recommend reading whole post to get the whole context) [source: link]:
And all this is not to say that copinglinking shouldn’t be a concept, but we need to rethink that as well. From the very few copinglink writings that exist, one topic I’ve seen several times is the idea of copinglinks becoming inseparable from you. This is not the point of links, and those who do go through a change like that find themselves more at home in the kin community than the link community. I don’t want to impose myself on linkers, but if we want these two words to make sense and have a use, we need to redefine both. I suggest defining copinglinks and otherlinks by their lack of integrality or by their ability to be dropped when necessary.
Otherflicker - temporary identification as something, It is common for otherflicker to experience 'flickers frequently. Most known are fictionflickers, which are temporary identification as fictional characters. I haven't seen the term 'otherflicker' mentioned much, though.
Otherfix - alterhuman identity caused by/influenced by/connected to hyperfixation.
Alterhuman - anyone who have an identity beyond what's typically considered being human. It includes many experiences.
Non-human - someone who doesn't identify as human (wholly or partially).
Definition of holothere from one of @/thetadelta posts [link]:
Holothere/Holotheric Therian: A physical therian who also identifies as nonhuman in all other ways, such as mentally and spiritually, and does not identify as human in any way.
Definition of plurality from pluralpedia:
Plurality is the state of having multiple headmates collectively sharing a single body. A group of headmates may be referred to as a system. Plural experiences are extremely diverse, and systems may vary in their origin, functions, distinction between members, member count, and internal communication.
Fictive - headmate based on a fictional source.
Definiton of soulbonding from pluralpedia:
Soulbonding is an intense connection someone feels to a character, which results in that character becoming a presence in their mind, either appearing in headspace or as an internal voice.
These characters are referred to as soulbonds, and those who have or gain soulbonds are known as soulbonders. Soulbonders may be considered a kind of host, or they may still see themselves as a singlet.
Nonhuman headmate - headmate that's nonhuman
Roleplayer - someone who roleplays.
Cosplayer - someone who dresses up as e.g. fictional characters.
Furry - someone interested in anthropomorphic animals
Feral - in the furry community refers to characters, fursonas, OCs, avatars that aren't anthropomorphic.
Otherpaw - someone who roleplays as an animal. May do quadrobics, dresses as an animal etc. [this term is pretty similar to feral]
Reddit post from r/otherpaw mods clarifying the definition of otherpaw [link]:
First things first, something we should make clear is that none of us at the mod team created the term. The rightful creator of the term is Nixyz_BTS🐾. Our wonderful discord mod, u/National-Sample-6148 managed to get in touch with her, discussing with her about the definition and symbol. This is the final result.
Definition: An otherpaw is someone who roleplays as an animal, usually by things like mask-making, gear, quads, etc. This is a voluntary identity so anyone can be an otherpaw
Symbol/Flag: The star with a paw inside it. This is the one Nixy created and so we will be going with that.
If I made some mistakes in definitions, feel free to correct me.