Parasite

The, called Parasite Chrysalis in the English dub of the anime, are a type of yōkai that possess other yōkai to protect themselves. Naraku uses Parasite Chrysalis to steal the Fuyōheki from Gakusanjin, and then ordered Kagura and Kohaku to exterminate them.

Sango knows very well the Parasite Chrysalis, and seeing the multitude of corpses in a pond, she understand that they have taken possession of a gigantic yōkai.

It is dangerous for a smaller yōkai to become the host of a parasite.