I fit had been a Bull Shark It would be nearly believable.
Bull Sharks can live in fresh water.
There’s also a freshwater shark found only in one Nicaragua lake.............
https://a-z-animals.com/blog/sharks-in-lakes-discover-the-only-shark-infested-lakes-on-earth/
Not really, there are about 8 hydro electric dams between there and the ocean. There is no way a shark could navigate all those fish ladders and what would even compel them to try?
You can buy freshwater sharks (the size of minnows) in any pet store, so they're not exactly rare or unusual. The sharks in Lago de Nicaragua were bull sharks that swam upriver and got landlocked a few hundred years ago. Now they're a separate subspecies. And they still eat people.
Bulls have been found in the Mississippi as far north as Alton, Ill. They're sort of like mollys (tropical fish), only in reverse. Under the right circumstances, they can acclimate to fresh water.