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Therian refers to a type of mammal, and is a word with a clear meaning within science and biology. But it has also been appropriated via a rather convoluted path in the late 1990s to gain a second meaning, referring to a subculture of people who think they're animals - and no, not in the taxonomical way.
A therian is a member of the Mammalia subclass Theria, consisting of marsupial and placental mammals. This subclass contains virtually all non-extinct mammals (including humans), the only exceptions being Australia's egg-laying mammals (Prototheria), the echidna and the platypus.
"Therians" are also people who believe themselves to be partly composed of some element of other living creatures' "personalities". This is similar to "otherkin" subculture.
Both of these terms (therian in the second usage, and otherkin) are very recent, and have been popularized by the internet. In 1996 the term "therianthropy" was just coming into use because some felt lycanthropy was wolf-centric and wanted something to include other species. "Therian" is a shortened version of "therianthropy", a term originally meaning the study of mythological human/animal "hybrids", but appropriated by the therian movement. The term "therian" became popular during the early 21st century.
There is, of course, no scientific explanation for how individual humans could share similar brain patterns or a "split-personality" with a vastly different creature, or how the "soul" of an animal (which is a non-verifiable concept in any case) could inhabit the body of a human being. While humans certainly share many of animals' primal instincts, most of the animals that therians claim to resemble have evolved along divergent paths from humanity and so cannot be regarded as having any kind of direct evolutionary influence on human behaviour. Some therians claim that they are perhaps suffering from a brain abnormality that results in multiple personality disorder, although nobody has bothered to investigate this hypothesis.
 Scientific pretensions
The only evidence that exists for therianthropy is entirely anecdotal and based on subjective measures such as "I feel like a cheetah in my instincts and behaviors." There are no established phenomena.
Therian subculture members have presented dozens of potential causes, ranging from the pseudoscientific to the pseudoreligious. These theories include:
- Appeals to mind over body, claiming physical transformation can be effected by mental powers
- Claims that the human genome contains genes of other animals which may reactivate, establishing traits.
- Something similar to multiple personality disorder, but with key differences.
- Species dysphoria (being born as the "wrong species", analogous to gender dysphoria)
- Occult explanations based upon animal spirits, shamanism, reincarnation, the astral plane, et al.
 Therian Shorthand
- "P-shifting": shapeshifting. Presumably, it should be staggeringly simple to prove this occurs via use of video recording. It is not a common belief among the therian subculture, though some younger and more gullible members believe it's possible.
- "M-shifting": claiming to take on the mindset of one's theriotype (i.e. an animal). The animal behaviors become more evident and the mental state becomes more primal.
- "Ph-shifting" (or "phantom shifting"): involves the alleged experience of phantom limbs associated with one's therianotype. (i.e. they feel as if they have a tail, or paws, etc.) Similar to someone who loses a limb and shortly after the amputation they still "feel" it there due to the fact their brain has not adjusted to the sudden loss.
 See also
- Breatharianism (Similar name but a different type of woo)
- Furry fandom