It’s no secret that Kozan is the native place for the ethnic youkai population of humans, and is considered Kozan’s indigenous people. Humanoids and youkai-borns are the two dominant ethnic groups in Kozan, both of which have shaped the Gensokyo continent’s vision to what it is today.
Youkai-born identity is a contested and complex issue. Thousands of youkai subgroups exists, and can be identified based on culture, history, language, religion, and mythology. In fact, some youkai species are only available in a certain gender. One big example is the Pix ethnic group, representing some form of nature. The only gender this group exists in is females. In order for that group to expand, they will either have to mate with a male person. If the offspring is male, they will inherit the father’s ethnicity, but if they’re female, they’ll usually inherit the mother’s ethnic group, but carry the genes of the father as well, making them a hybrid between the two. Some females can become Pixes if their mother isn’t one, but this may only happen if the father is carrying the genes of a Pix.
There are some other youkai ethnicities that are female exclusive, such as the yuki-onna, but I’m sure there’s more than that.