Variations: Hantu Kotak (Malay), Box Spirit

Bès Kotak, “box spirit”, is a bès or spirit from the folklore of the Jah Hut people of Malaysia. It is apparently box-like in appearance and is a river spirit that lives in the muddy hollows of rivers.
When a person dives into the river to catch fish, the box spirit presses or sits on that person, causing them to become heavy, sink into the mud, and drown. Two or three days later the body will float to the surface, proof of the fate that awaits any who invade Bès Kotak’s domain.
References
Werner, R. (1975) Jah-hět of Malaysia, Art and Culture. Penerbit Universiti Malaya, Kuala Lumpur.