Okay, due to this being a topic that is blown off by most people on here because 'it's fluff'. We're here to tell you, guess what? Therians are not fluff.
True therians (Ex. not 5-year olds who say they can p-shift) know they cannot p-shift. Therians believe they have the soul or spirit of an animal. The animal normally must be physically existing, because if it is not then they are called otherkin (Hence why dragon otherkin exist, but dragon therians are not truly therians.)
Therians often experience phantom limbs, otherwise known as astral or aural limbs. Things that exist on their body on the astral plane, but not the physical plane. Therians also experience mental shifts, something that is also widely blown off as a 'fluff' topic. Mental shifts, in reality, are literally just changing they way you think. Some therians meditate on it, whilst others it occurs naturally. It is the same thing as a cat being raised around dogs, then the cat acts like a dog.
Otherkin often experience the same things, but for some it's on a lesser sense. Mental shifts may not happen, and phantom limbs may only be partial or only occur when triggered by something such as strong wind (Wingedkin, birdkin, dragonkin, ect.)
Otherkin are not the same as therians as therians believe they have the spirit of that animal, whereas otherkin believe they were that animal in a past life, not entirely that it is their soul shape/soul type.
Some otherkin, however, use their identities as coping methods for mental illnesses. Some, however, truly do believe they were that in a past life, and it is no one's grounds to tell them they are not or never will be.
TL;DR: Therians and otherkin are not fluff, and never will be.
The reason people write off otherkin and therian is because it purely a belief, until proven false or fact, we can never truly know. There is no real evidence to suggest that otherkin or therians are real, things such as phantom limbs and mental shifts can be attributed to many things such as, Mental disorders, Creating it Themselves Through Want, Social Isolation. I have nothing personal against otherkin, but I'm just giving the facts.
Can any belief truly be proven, however? Based on the logic you use, that can be applied to any belief, such as magic, religion, literally anything spiritual.
But yes, some do use it as a coping method for mental illness, whereas others swear they feel phantom limbs. We are a therian, and we will tell you we feel the limbs too, but whether you believe us is your own choosing.
That is definitely true, but you are saying that your specific belief is true. I do not say that my beliefs are true to other people, because people have there own beliefs. Simply stating that your belief is true is an act of ignorance, but of course some will believe and some will not.
I see points on all sides of this topic. I personally believe i am an otherkin and wish to explain. Upon seeing and sensing an aural form of a black wolf, which then chose me for her to be my familiar, i noticed that over time, whenever my third eye is open, it feels like my senses have gradually become stronger. This is in an aural sense only. Not physical. My eyesight is keener. My sensing is icreased. I seem to think more strategicly as well as logically. And also when i look into a mirror i see myself as i am physically but i also see a hazy outline of wolfish features. Its almost similar to describe it as a blur. And also other times i feel and see black wings which adjoin to my body between my shoulder blades and spine on either side. Those seem to be at full length about a twelve foot span. Not sure why this is but hey i guess it is what it is.
I believe wholeheartedly in Otherkin and Therians. I believe I am a Dragon Otherkin, I do not have any Therian in me that I know of. How did I come to believe this? One, I instinctively grasp Dragon Magic. Two, I feel phantom horns, wings, claws, scales, and even a tail. Three, the Dragons I work with see me as such. I don't believe I can take the bodily form of a Dragon or breathe fire, though that would be really cool. Humans having the spiritual essence of another being is by no means impossible.