Spirits

At the beginning, there were the Spirits. It's not clear whether they shaped themselves out of the aether to resemble animals or whether the Spirits themselves created animals in their own image. Monks call them the first children of the Divine. Even the natural philosophers will grudgingly admit that some evidence suggests powerful aether-using creatures existed before the Human age.
The most well-known Spirits of folklore resembled wolves and snakes (terrifyingly powerful), owls and cats (wise but uncaring), foxes and rabbits (mischievous though not evil), and horses and cows (good natured and helpful).

Basic Information

Genetics and Reproduction

Spirits are capable of changing themselves into human shape. There are usually some clear tells that distinguish a Spirit from a mundane human, such as brightly colored hair, markings, etc. While shifted, Spirits can (and very often did) engage in sex with humans, leading to descendants of mixed heritage able to touch the aether--the first Ninja.

Additional Information

Uses, Products & Exploitation

Almost every part of a Spirit's body resonates powerfully with the aether. Unfortunately for those seeking to get rich quick off this fact, Spirits mostly dissolve back into that aether when they pass. In rare circumstances, however, that can be blocked by certain rocks or runes and seals known to ancient humans.

Conservation Status

Ask a merchant or a lawyer, and they'll tell you Spirits never existed. A fairy tale. Ask a farmer or a woodsman, and they'll tell you the Spirits are gone. Faded away. Get a ninja very drunk, and they'll tell you in a whisper the story that the scholar Rayan Itani was killed for publishing. That humans--that ninja--killed the spirits, hunted them into extinction in their lust for power.
All of them are wrong.