When H Melville stuffed the middle of Moby Dick with a "cetology" -- BEFORE The Origin of Species, famously saying "a whale is a fish" -- he didn't forget the Greenland Shark. I think all the time about how many of those sharks swimming around in 1851 are still swimming around today.
There's a business lesson in the longest lived creatures being the ones that move slow, abide small insults, and make themselves generally unappetizing.
Jeremy Wade, host of the TV show "River Monsters", has an episode where he investigates the Loch Ness Monster and concludes it's likely a Greenland Shark that swam up an underground river from the North Atlantic to the lake. He likens the shark's horse-like face and the distribution of the low fins on the shark's back to descriptions of the monster. A solitary long-living fish could explain the occasional sightings, and the scientists' findings that there is not enough food in the lake for a breeding population of large carnivores.
As a sibling comment notes most sharks cannot live long in freshwater, and moreover this is soecifically true of Greenland sharks, though they do sometimes spend time in brackish river mouth environments, so, unless it developed the weird behavior of migrating quickly up the relevant underground river to make a quick appearance and then inmediately rushing back down the river to the ocean, that’s one answer we can be fairly certain is wrong.
There are a few sharks that can live in freshwater, but they tend to inhabit warmer oceans.f_
He is likely wrong (most sharks cannot live long in fresh water). But given the show, he has to conclude it is a fish of some sort, and it is not going to be 10k arctic char in a dinosaur suit.
See also https://news.ycombinator.com/item?id=46511555
There are a few sharks that can live in freshwater, but they tend to inhabit warmer oceans.f_