Parasites (宿り蛹, やどりさなぎ, Parasite Chrysalis, "Yadori-sanagi") are a type of yōkai that possess other yōkai to protect themselves since They are weak by themselves. It is dangerous for a smaller yōkai to become the host of a parasite.
A lot of them had made a nest inside of the giant yōkai Gakusanjin. When Naraku stole the Fuyōheki from Gakusanjin, he the parasites lost their nest and started plaguing the entire region. One of them was able to possess a Fox Deity and had it terrorize a village. Another later possessed Shippō. Inuyasha and his friends later came across a pond that was full of dead parasites. Miroku sucked them up with his Kazaana
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.