Leviathan

Not-fish are usually not much of a problem. They aren't interested in humans and their biggest threat is just passing on their infection if you eat them.
Aberrants, however, can cause some problems. In rare cases, a fish that becomes infected by bloodsap (usually by eating an insect like a mosquito) mutates in unexpected ways. They can become enormous, with new limbs and unprecedented abilities. Not two aberrants are the same, but all aberrants are deadly.
Any water-dwelling creature that becomes an aberrant is called a leviathan, and their appearance is always a crisis. In the summer, they pop out of the water to snatch sailors off the decks or ram their hulls to sink the ships. In the winter, they smash through the ice and send travellers crashing into freezing water, just to pick them off below. Anytime a leviathan appears, a nation's military is called in to destroy it.
Luckily, leviathans are rare. There have only been a couple dozen confirmed cases in the 125 years of the Forest. Improved scouting in recent decades has led to leviathans being discovered before they grow to massive sizes.
However, rumour has it that a particularly massive leviathan in Lake Taiha has yet to be destroyed. In the deepest and largest lake, there is a lot of space for a creature to hide. These rumours have been combined with old folklore about Tailalli, an ancient monster said to inhabit Lake Taiha.