First and Foremost I want to start of with saying that everything talked about below is purely taken from my own experiences and observed information from my surroundings and those that I've talked with. I do not claim to be an all knowing source of information on these topics, however I haven't seen any topics about these two subjects on this site.
If you have questions I will gladly answer them to the best of my abilities. First, lets get the definitions out of the way. Again this is what I have observed.
OtherKin: a sub culture of people who identify as partially or entirely non-human. ex: part angle, dragon, faeries, etc.
Therianthropy: mythological ability of humans to shape-shift into animals.
Therian: someone who has connected with previous past lives, usually those of animal form. ex: wolf, coyote, tiger, woolly mammoth, etc.
Reincarnation: the belief that a human, or any life form, can have its soul re-birthed into this world as another creature after death.
So lets start with an OtherKin. OtherKin, like described above, is a sub culture that exists online and in real time. Their believes usually lie within the realm of fantasy, and or role playing, etc. Though, I believe this site also would classify them as fluffs. Members that are new to the craft and aspects of the craft.
Those that have these views strongly believe they are what they are, and honestly I'm not going to tell them otherwise. However, I do believe in channeling aspects like angels, dragons, faeries etc into our practices. This I can see would make them feel a lot like they are connected to you, and or are a part of your being. That to me, makes sense. Yet, I'm skeptical or those that belief they are part or completely non-human in this life.
Moving on. I won't go over Therianthropy, because honestly, that is such a large topic that includes folk lore on werewolves, weretigers, etc that I just don't feel like going over it all. There are plenty of research out there on the internet and in libraries for you to look into that would go into it in much more detail then I ever could. Most of these explaining where these believes came from, how they can effect today's life, and how there is a bit of it that makes sense. Like channeling an animal's life force, aura, energy to help within everyday practices. Like picturing yourself as a bear, or your surrounding energy as a bear when faced with a fearful event or obstacle. Energy is all around us, and it's all about how you use it and channel it that makes a difference.
Now, to a Therian. A Therian, like described above, is basically someone that has made contact with an animal based previous life. It is really not all that different than meditating and looking within yourself for a past experience that this life wouldn't remember after being reincarnated. There are also some terms that Therian's like to use from time to time, and I have even used myself. Awakening, is a term used to describe the moment when connecting to one of these past lives. I can use my own experience when I connected with a past life. I call this life Areyia. It, or rather she, happens to be a Smilodon Fatalis. Now in common terms she, would be one of the classifications of a saber tooth cat, or saber tooth tiger. Connecting to one of these past lives, for me at least, was much like connecting to a guide, guardian animal spirit, and/or making contact with a part of Inner-Self. Another term often used by Therians, and OtherKin, is a phantom limb(s). Now this phenomenon also occurs rather regularly with amputees and people that have lost the ability to feel their limbs (through nerve damage, etc.). For Therians, and OtherKin, this often occurs as the feeling of having extra limbs (like a tail or wings) where non exists, or having the feeling of having paws, talons, claws, etc rather than having hands. I could go on with a list of what these phantom limb(s) can entail but I won't. These feelings are just as real as your ability to feel your own arms and legs, even if they don't exist in the physical realm.
(This is where it starts getting tricky because I don't actually know that much about astral projection, or the astral body). My hypothesis and thought process on this phenomenon is that these phantom limbs are indeed there; just only as part of your astral body. Or even just a part of your surrounding aura/life force. In the past lives aspect, it's your soul remembering what it feels like to have these limbs. What it felt like to have these muscles and attachments you don't have in this physical body. Just like how it is with amputees, where their mind remembers what it feels like to have that limb attached; it's still a part of the astral self, but not the physical.
And that is where I will end my little spew of information. Again, if you have any questions or wish for me to elaborate or explain more just ask. Thanks so much for reading, and I apologize if it doesn't seem all that put together. I was pretty late when I was writing and proof reading this.
Otherkin, and Therians