Kawa-Akago (JP: 川赤子) aka the "river baby"
•About this Yokai: a sneaky child like kappa cousin who's modus operendi is mimicing noises: in particular it makes cries, sounding exactly like a human baby. - it hides under the reeds by the shores, cries & then when someone tries to save what they think is a drowning child, it pulls the good samaritan under, often drowning them instead.
• History and early appearances: Kawa Akago appears in Toriyama sekiens Konjaku zoku Hyakki. (1779) - it might be related to other yokai such as kappa or the similar, yama-akago from akita prefecture. (who hides under piles of leaves doing the same thing.)