Moon people, people of the moon
The Lunarians (月人 tsukibito, lit. "moon people"), also 月の民 (Tsuki no Tami, lit. "people of the moon") are the people living on or coming from the Moon. The term refers to a specific species, but it can also refer to any creature from the Moon in general (such as moon rabbits and gods).
Lunarians in Touhou
Thousands of years ago, the original Lunarians consisting of Lord Tsukuyomi and his most trusted relatives, including the older Eirin Yagokoro, came from the Earth to the moon. They were able to do so because they had the revelation that the world was built on probabilities and anything could happen, allowing them to artificially increase the probability that living beings on Earth would find their way to the Moon.  Their reason was to leave the impurity that was ravaging the land.  It somewhat implied that the original Lunarians were human, as the Lunarians resemble humans in most ways to the point where they can live as humans (functionally speaking) on Earth, and are classified as human in Perfect Memento in Strict Sense (however, the author Hieda no Akyu does not seem to be aware of the extraterrestrial origins of the people being classified as such). However, Lunarians are closer in nature to hermits or celestials than normal humans. They naturally live much longer lives, and are even said to live forever; they are not true immortals, however, and may die eventually due to unnatural causes such as accidents or battles.  Lunarians also allegedly have superhuman stamina and abilities beyond even youkai of the Earth, and possess some of the most advanced technology that there is. 
Supposedly, according to Kaguya Houraisan, the Lunarians not only are the originators of most powers on Earth, but also turned some earthlings into monsters in order to regulate the Earth's impurity. However, Yukari Yakumo has an alternate origin of youkai, saying in narrative to herself that it was the moon's dim light which gave birth to the youkai in a manner of speaking, by causing humanity to fear and create legends about them.  The two explanations are not entirely mutually exclusive, and may be a result of differing outlooks or parts on the matter. For example, the Lunarians may have purposefully dimmed the moon's light to make humanity's fears create the youkai.
The Lunar Capital appears to have a mix of Chinese and Japanese architecture and dress styles, but the extent of the connection between Lunarians and these cultures is unknown.
Characters under this Bestiary
- Resides in the Lunar Capital
- Lord Tsukuyomi
- Watatsuki no Toyohime
- Watatsuki no Yorihime
- Sagume Kishin
- Moon Capital Gate Guards
- Resides in Gensokyo
- Lunar Capital
- Jewelled Branch of Hourai
- Moon rabbit
- Wikipedia entry for Extraterrestrial life
- Cage in Lunatic Runagate: Third Chapter — "The Lunarians had long ago realized that our world is built on probabilities, that absolutely anything can happen. That was what enabled them to move from the Earth to the moon in the first place."
- Cage in Lunatic Runagate: Third Chapter — "Lord Tsukuyomi took only relatives he had the utmost trust in and moved to the moon." and related text around it.
- Cage in Lunatic Runagate: First Chapter - "I decided I would atone for my sin by helping find a place on Earth where she could live an easy life as a human."
- Symposium of Post-mysticism: ZUN interview
- Cage in Lunatic Runagate: Third Chapter — "Of course, since the lunar citizens and the moon rabbits aren't immortal, they can die in, for example, accidents or battles. Still, even the citizens of the lunar capital bear just the faintest trace of impurity. Even we may be fated to die some day."
- Cage in Lunatic Runagate: Fourth Chapter — "The Lunarians possess highly advanced technology, unyielding stamina and abilities that are far outmatch those of youkai."
- Imperishable Night: Scarlet Team's Scenario — "The moon granted many powers to the Earth. Most powers such as magic were originally those of the moon". "We of the moon turned some on the Earth into monsters, to regulate the Earthlings' impurity."
- Cage in Lunatic Runagate: Fifth Chapter — "Under the dim light of the moon, all sorts of things blanket the Earth in shadows, such as grasses, trees, mountains, rocks and buildings. With light as weak as moonlight, it is impossible to eliminate these shadows. And they are the birthplace of humanity's fears, sometimes creating legends of youkai. In a manner of speaking, the moon gave birth to all youkai."