LOL "Harpy Marriage"
I went crazy with the shiny. I'm sorry.
Most of the Japanese yokai monsters are only shown to be one gender (Unlike the Nordic monsters who more often than not has big families) but for some reason especially the tengu's private life has been the source of great speculation. None of the old stories explains where tengus come from, they are always male and there are no stories of them having intercourse with human women, so it has been speculated that the tengu women look so different that they are considered a different race, or possibly that they live far away from the men.
I joined the two theories with the harpies. They are only female but are said to get pregnant "from the wind", and tengus are said to be the masters of wind.