ANSWER: A skate (홍어 洪魚)?
The following comes from a 1668 entry in the "Annals of King Hyeonjong" (현종실록):
In the Nampyong region of Jeolla Province (全羅道 南平地 전라도 남평지), someone (有人 유인) gave birth to (生 생) a girl (女 여) on whose left buttock (尻尾左邊 고미좌변) was a growth (生贅肉 생췌육) that measured one foot, one-and-a-half inches in length (長布尺一尺一寸五分 장포척일척일촌오분) and was shaped very much like a skate (酷似洪魚狀 혹사홍어상), according to the provincial governor (道臣以聞 도신이문).
Here is a picture of a "skate" (洪魚 홍어): [Source https://en.wikipedia.org/wiki/Skate_(fish)]
Do they look very much alike? Couldn't the growth that looked very much like a skate on the left buttock of the little girl have been just the mother's placenta? Wouldn't it have just peeled off? Or was it really attached in some way?
