“A therianthrope is a person who feels a connection to a particular type of animal so strongly, s/he feels that s/he is, in fact, that animal in most ways except physical. This is an extremely powerful feeling that in most cases began at a very young age. It is usually accompanied by a strong desire to become that animal, lots and lots of instinctive behaviors, thoughts, and desires that echo the natural behavior of that animal--and sometimes, mental and emotional ‘shifts’ into the specific mindset of that animal. [...] We do NOT physically turn into animals (though the Universe knows we’d like to). [...] not all therianthropes ‘shift’, voluntarily or spontaneously. Some simply feel like an animal all the time.”
- Excerpt from “Therry-what??” by Ozenwolf, written in 2002. Read the full article here.
One definition of therianthropy uses some form of the phrase “In all ways except physical, I am an [animal].” Ozenwolf (he/him, wolf) and Swiftpaw (she/her, jaguar) both used the phrase in their essays from around 2002, as in this excerpt. That means the phrase is at least twenty years old, as of this writing. I don’t know who invented the phrase or when.