Probably the most notorious thing on the show is the Goonch, aka the Giant Devil Catfish. It's a massive, predatory catfish, well over six feet long, that lives in the cold, murky waters of the Kali River in India. Unlike many catfish, the Goonch has sharp, almost shark-like teeth◊ and is a free-swimming predator rather than a bottom-feeding scavenger. There are reports that it will attack and devour humans, but what really takes the cake is how it may have come to view humans as food. The villagers on the Kali cremate their dead, and then push the burned bodies into the river. These fish have potentially been feeding on burned corpses for centuries, and that just may have given them a taste for live humans as well...
Also, the one Jeremy caught is nearly six foot long and weigh 170 pounds, and it's still not big enough for a man-eater, meaning that the ones that hunt swimmers are still out there.
Many of the stories told about the fish, which give them their fearsome reputations, are these. The giant catfish in the Amazon that was caught with a fisherman's corpse jammed in its throat. The goliath tigerfish attacking and killing a little girl. The wells catfish swallowing ducks whole, and medieval accounts of them having been found with children in their stomachs...Trust us, it goes on. Fortunately, most of these are exaggerations. Most.
Blood sucking primordial fish invading freshwater rivers and lakes, dangerous to even the most powerful fish, and not to mention humans? As a New Englander, I find Sea Lampreys to be terrifying.
What Jaws did for the ocean, he's doing for every river and lake he can find. Seriously, you know a show has a pretty high dose of Nightmare Fuel when piranhas are among the less dangerous creatures featured.
Extra Jaws shoutout goes to the Bull Shark episode. Already not too much smaller than Great Whites, and meaner to boot, they're able to operate just as well in fresh water, the only shark in the world to do so. They've been found in sea-connected rivers up to 2200 miles inland.
And it's the big ones that tend to visit rivers; the larger ones can handle the stress better. In fact, the biggest bull shark ever caught anywhere was caught in a river.
Oh, and a bull shark was the actual species of the shark whose series of attacks were the inspiration for Jaws.
The New Zealand longfin eel, star of the season 3 episode "Flesh Tearers". While the 8ft long or bigger eels aren't confirmed as existing, they are aggressive enough that they can and will pack-swarm even humans and rip them apart.
"The Mutilators" features the pacu, a vegetarian relative of the piranha that's been introduced to Papua New Guinea. It turns out that, far away from the Amazon, this fish with its crushing jaws and disturbingly human-like teeth has turned omnivorous, with a particular penchant for biting the testicles off of any man it can meet in the water. What's worse is that, as introduced species tend to do, it's multiplying beyond the control of the native species and is driving them to extinction.
The Tigerfish. The Goliath Tigerfish from the first season is big enough that it can bite a small child almost in half, aided by its demonic sharp teeth. But even the smaller species that lives in South Africa are deadly. They're pack hunters, like piranha... but tigerfish can be up to 3 feet long and weigh in at dozens of times the weight of the inches-long piranha.
The Vundu is an actively predatory catfish species from the Zambezi river. Giants capable of eating humans are rare, but a healthy-sized adult is more than big enough to gulp down a human toddler, something they have a well-earned reputation for doing...
An episode centered on Japan speculates that the kappa may actually be a mythology-wrapped representation of the Giant Japanese Salamander, which is nightmare fuel on two levels. Firstly, its method of feeding is to inhale prey straight into its stomach to digest them alive. Secondly, it gets big enough that it can eat human babies.
The catfishes of the Congo River. More accurately, the kill or be killed lifestyle there shared together by fish and people.