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.
We had two. They grew to be about 8 inches long, then jumped out of the tank one night.............