The enormous goonch, a type of catfish, is said to have developed a taste for human flesh after feeding on corpses thrown into the river after funeral ceremonies.
Locals rumours have held for years that a mysterious monster lurks in the water.
But they think it has moved on from scavenging to targeting live bathers who swim in the Great Kali, which flows along the India-Nepal border.
The extraordinary creature has been investigated by biologist Jeremy Wade for a TV documentary to be shown on Five.
He said: "The locals have told me of a theory that this monster has grown extra large on a diet of partially burnt corpses. It has perhaps got this taste for flesh by feasting on remains of funeral pyres.
"There will be a few freak individuals that grow bigger than the other ones and if you throw in extra food, they will grow even bigger."
Mr Wade caught one goonch, among the largest freshwater fish in the world, which weighed 161lb and was nearly 6ft long – a world record for the species.
He said: "If that got hold of you, there would be no getting away."
In 2007 an 18-year-old Nepali disappeared in the river, dragged down by something described as like an "elongated pig".
However, the first live victim of a goonch was thought to have been a 17-year-old Nepalese boy in April 1988.
Witnesses said he was cooling himself in the river when something suddenly pulled him below the surface.
Three months later a young boy was dragged underwater in front of his horrified father.
Five's Nature Shock series starts on October 14 and Flesh Eating River Monster is on October 21 at 8pm.
Dan ini bukan mutant loh, tidak ada perubahan gen abnormal atau lainnya, tetapi hanya perubahan perilaku. If there is an ecological niche, sooner or later something will adapt to it. So this is hardly surprising.