Classic of Mountains and Seas
A "ningyo" might be counted as a "yōkai" since it is included in Toriyama Sekien's "Hyakki Yagyō" series. A particular gender is not clear here, as it is only given as a being with "a human face, a fish body", and Sekien equates the "nigyo" to the Di peiople or , which are described in the "Classic of Mountains and Seas" and translated as the "Low People".