Traditionally speaking, a therianthrope is a creature of myth and imagination. The word is derived from therianthropy, which comes from the Greek words therion, meaning beast, and anthropos, meaning man. In common parlance, therianthropy can refer to a few different things.
The first definition has to do with the ability of some humans of legend to shapeshift into animals. Werewolves, werecats, selkies, and wendigos are primary examples. The second meaning refers to a state in which a deity or other supernatural creature may be part man and part animal in appearance. Gods like Anubis and Horus, as well as creatures such as mermaids and centaurs, can all fall into this category.
Therianthropy also has more specific branches. Lycanthropy which encompasses werewolves and all other versions of wolf-men, cyanthropy which refers to men who can change into dogs as commonly found in Chinese myths, and ailuranthropy which pertains to shapeshifting humans who can take on feline appearances, are all forms of therianthropy.
The word therianthrope, however, is often used outside of this traditional context. In some New Age communities for example, a therianthrope (also called therian) is a person who subscribes to the neo-pagan belief that he or she is part animal. A New Age therianthrope does not actually have the ability of metamorphosis and will not exhibit evidence of animal body parts in his natural, surgically unaltered state. Instead, he believes that as a therianthrope, his spirit is part animal. The New Age therianthrope also claims that he is in possession a certain kind of animalistic energy which differentiates him from normal human society.
In a therianthropic community, closeness to particular animal species, "primitive" emotions, a deep appreciation of nature, and a different, more instinctive world view are all seen as evidence of therianthropy.
There have also been documented cases of clinical therianthropy, where a patient may develop delusions of being part animal, or shapeshifting into an animal. These delusions are often associated with deep psychosis.