Modern Therianthropy is an identity phenomenon categorized by a deep integral or personal belief that an individual is in some way and to some degree, a non-human animal. Physically, Therianthropes are human and understand that they have human bodies.
"Therianthropy is a state of being, whereas someone believes an animal is an essential constituent in their spirituality or life. Therianthropy is an innate part of the person's life. Therianthropy is the recognition and acceptance of the animal within. It makes us what we are, but more importantly who we are.
Because there seems to be some confusion over what a therian is and is not, therianthropy is about the inner nature of one's own self. Therianthropy is about the discovery of the animal within, that we are born with. You cannot become a therian, you must be born that way. While we are not stressing that therianthropy is the only kind of personal animal philosophy/spirituality, we are stressing the differences BETWEEN therianthropy and these other 'ways of life' or spiritual beliefs." 
Most Therians identify as earthly animals whether currently living species or extinct species such as dinosaurs. There is not a hard distinction, however, and individuals with mythological or non-earth animal identities may have experiences that are best supported by the Therianthropy community, such as individuals who identify as dragons. Or they may find more support or feel more at home in the Otherkin community.
The majority of Therians are private about their non-human identity and keep more animal-like behavior under control while in public. The ability of Therianthropes to control their actions during shifts, when they feel and act more instinctual and animal-like, is what sets Modern Therianthropy apart from mental disorders such as clinical lycanthropy.
Therians don't know what causes them to identify as non-human. Therianthropy is a subjective belief based on personal experiences and not on scientific fact, with most Therians being "self-labelled" or "self-identified". Therefore, both Spiritual Therianthropy and Psychological Therianthropy are legitimate and acceptable in the community. Their experiences, such as shifting, is essentially the same. The majority within the community agrees that Therianthropy is not a choice. It is subconscious or unconscious.
"Therianthropy is thought by some to be present from birth or acquired in very early childhood because most Therians do not have a memory of ever having not been Therians (though there may have been a period of not recognizing the nature of their Therianthropy)." - Therian Timeline 
It is also agreed upon that a Therian can not pick and choose a theriotype, the animal which is an individual's identity. Some individuals enjoy the experiences associated with Therianthropy, some do not, and others are indifferent to being a Therianthrope.
Skeptics might argue that identifying as Therian occurred after finding the online community, but Therianthropy is not a group identity. It is a personal identity with the majority of Therians feeling non-human and/or exhibiting species-specific behavior before finding and participating with the online community.* Therians often share memories from childhood of having always experienced a vague feeling of being other than human.
*People without a previous history of therianthropic-like feelings/experiences who do identify as Therian AFTER coming across information about it (commonly on Youtube, Tumblr, Instagram, and Facebook where inaccurate information is shared) are usually met with more skepticism and questioning on more well-known and established forums. They may be suffering from what is known as Shifter's Disease.
- See also: Common Misconceptions
As the online community grew, some people felt the need to coin new terms for different or varying experiences.
The word Therianthropy originates from a portmanteu of Greek words. Thirío [θηρίο] or thēríon [θηρίον] means beast or wild animal. Anthropos [ἄνθρωπος] means human or person. Loosely translated to "animal person".
In historical contexts the term therianthropy is used to describe people in mythology and folklore who could change shape and become were-creatures. One book raises the possibility that the term Therianthropy may have been used as early as the 16th century in criminal trials of suspected werewolves.
"Between 1521 and 1600 several men were prosecuted in court for "lycanthropy" or therianthropy (becoming a wild beast), and such cases were recorded into the eighteenth century. In fact, during one extended period some thirty thousand cases of werewolf possession were reported to authorities. In God's name, inquisitors sometimes hacked these people apart to search for the telltale wolf hair supposedly planted inside their bodies." 
There are currently no documented cases of the term being used to describe similar situations to that of current, modern day Therians, but there are many well documented examples of what could be considered Therianthropy.
One of the first known written and published appearances of the word therianthropy can be seen in "The Religious Systems of China" written by J.J.M De Groot in 1901. The usage of therianthropy can be found on page 171 of Volume IV, Book II "On the Soul and Ancestral Worship, Part 1. 
Therianthropy was also used to describe spiritual beliefs in animal transformation in a 1915 Japanese publication, "A History of the Japanese People from the Earliest Times to the End of the Meiji Era."
The term has often been used by archaeologists to describe animal-human figures found in prehistoric rock art such as the "Dancing Sorcerer" at Les Trois-Frères in southwestern France. Theriocephaly refers to beings which simultaneously share human and animal traits, such as Egyptian Gods.
The word "therian" also has a clear meaning within science and biology. Therian, in taxonomy, means a member of the mammalian subclass, Theria, consisting of marsupial and placental mammals and their extinct ancestors.
Modern day usage of the word is thought to have originated in the early 1990's, when users of the werewolf newsgroup Alt.Horror.Werewolves (or A.H.W.W.) began discussing their spiritual and mental connections with animals, asserting that they were in fact their therioside.  At first, the term Lycanthrope or Werewolf was most commonly used, but Therianthropy became more widely accepted as it catered to all species, and not just wolves. A.H.W.W. is credited the birthplace of the online Therian community.
- Proctor, Devin "On Being Non-Human: Otherkin Identification and Virtual Space" The George Washington University, ProQuest Dissertations Publishing, 2019. 13810285. https://search.proquest.com/openview/e156c24bf65c4efb0918a8db37433cce/
- Bricker, Natalei, "Life Stories of Therianthropes: An Analysis of Nonhuman Identity in a Narrative Identity Model" (2016) http://publications.lakeforest.edu/seniortheses/63/
- Grivell et all, "An Interpretative Phenomenological Analysis of Identity in the Therian Community" (2014) http://www.tandfonline.com/doi/full/10.1080/15283488.2014.891999
- Shifters.org Encyclopedia - http://web.archive.org/web/20060821231231/http://www.therianthropy.org/SO/terms-t.htm
- Therian Timeline, Mind of the Were, Development - http://theriantimeline.com/therianthropy/the_mind_of_the_were
- Ramsland, Katherine, The Human Predator: A Historical Chronicle of Serial Murder and Forensic Investigation Berkley Books, 2005, pg 30. https://books.google.ca/books?redir_esc=y&id=VtduoGffUgEC&q=therianthropy#v=snippet&q=therianthropy&f=false
- “The Soul in Philosophy and Folk-Conception Willful Change of Men into Beasts.” The Religious System of China, by Jan Jacob Maria de. Groot, volume IV, book II, Brill, 1892. https://archive.org/details/religioussystem04groo/page/170
- Brinkley, F. History of the Japanese People from the Earliest Times to the End of the Meiji Era. Project Gutenberg., 1855. https://archive.org/details/historyofjapanes00briniala/page/n6
- "Dancing Sorcerer" at Les Trois-Frères - http://www.ancient-origins.net/history/shamanic-explorations-supernatural-realms-cave-art-earliest-folklore-007027
- "Foundation's Edge: The History of the Online Werecommunity, Jakkal, 1999. http://web.archive.org/web/20010620111439/http:/www.were.net/foundation.shtml