You will have to do the research but their are reports of a few in the Mississippi and the Ohio. Also a couple other saltwater species that can go up freshwater. Whether it's true or not it is within the realm of possibility.
I have caught a lot of giant catfish and snagged huge spoonbill. I know what a big freshwater fish feels like. I also have saltwater fished some and know what a decent saltwater fish feels like.
I was fishing on the Ohio River some years back near Mound City with a big chunk of shad...had something grab it and head out ...nearly spooled me with a penn 309 and 50 lb test before it cut me off...wasn't like anything I ever felt before in fresh water...who knows
This goes hand in hand with the Bigfoot stories. Until one is captured there will always be doubt.
As they say, proof is in the puddin'...