If anything, being exiled by circumstance to an island tends to lead to insular dwarfism, which is kind of the opposite of King Kongification.
Not to mention a coelacanth doesn’t look as different from any extant fish in either size or appearance as a marine reptile does. A layman is likely to notice that “fish” has a long neck.
His ancestors were much bigger monkeys. You Kongification deniers are all alike.
His ancestors are hiding in video games. It’s on…
Coelacanth–mediumish fish that looks like a fish, lives surrounded by hundreds of other species of fish. Sauropod–reptile the size of an apartment duplex, can eat a forest on a lazy afternoon. See a slight disparity in overall noticeability?
The OP’s question was whether surviving dinosaurs might be the inspiration for the worldwide legends of dragons. So a small population of dinosaurs in a remote location wouldn’t qualify.
Elephant birds survived in Madagascar until the arrival of humans, and may possibly have inspired legends of the roc.
Well, what’s a “modern human”? I mean Homo Sapiens saw and ate Mammoths, Megatherium, and many other ancient extinct giant land mammals.
Not to mention Moas and Aepyornis which are technically dinosaurs and very large. The last Aepyornis may have existed until about 1000 BP.
There have been rumors of truly huge Nile crocodiles.
Large monitor lizards still do exist and larger ones may have been hunted by man (or the other way around).
There’s also a recent sci-fi thriller book called “The Great Zoo of China” that the main scientist character postulates that dragons are dinosaurs that had eggs that could “hibernate” for centuries in the event of non-favorable conditions, thus all the stories of dragons were just these eggs hatching once every few decades from some remote cave that still housed them. The reason why there was a sudden drop-off in dragon sightings after the middle ages was because most of the remaining eggs had already hatched.
Then China finds a still viable batch of eggs, uses genetic engineering to clone them and make their own “Jurassic Park” style animal park to show them off. Then the dragons escape. Hilarity ensues.
Just because it has been alluded to but not called out directly and this being Genreal Questions.
Birds are theropod dinosaurs, and birds the only dinosaur group that are known to have walked the earth at the same time as humans.
Komodo dragons are, even now, not actually limited to one small island; despite the name, they are not just found on the small island of Komodo. They’re actually found on several islands, including Flores, which is over 200 miles long.
They are the largest extant species of lizard, specifically a monitor lizard (which are a very widespread group of lizards), not a dinosaur, as has already been said. They may well have been found over a much larger area before being restricted to their current range. Fossils very similar have been found in Australia, as well as many more small islands.
I don’t know medieval knights would even notice a dinosaur- they were too busy in their war against GIANT SNAILS.
An air breather hiding in the depths somewhere. Let me ponder upon that…
I didn’t know there were books about his key lime pie recipe! :eek:
Veterinary science–when will Mankind learn! 
On the subject of the legend of the roc, Adventures in Unhistory suggests that sailors may have seen huge domed buildings at a distance and thought they were the eggs of an enormous bird.
Yeah, who would have thought?
Again, these aren’t all that distinct from any other cetaceans to elicit much notice, particularly from laypeople.
Ichthyosaurs are close enough in morphology to dolphins I can almost buy they’d go unnoticed. Any other marine reptile not so much.
Dodos were as ancient, if not more so, than mammoths when they went extinct. Also, “modern humans” have been around for about 200,000 years, so all the extinct megafauna we are familiar with from that time was around. Perhaps you meant “civilization”. But again, there is nothing particularly ancient about wooly mammoth as compared to other species that have gone extinct in the last 5,000 years or so.
Ok, I can agree to that. Kronosaurus and other Plesiosaurs probable not, but it is curious how much “sea serpents” look like a Plesiosaur.
Mosasaurs are more likely, since many Ichthyosaurs went extinct before the K-Pg event .
Here is a pic of many of the ancient marine reptiles:https://www.pinterest.com/pin/369717450645464659/
and various types of mosasaurs:
How old are Plesiosaur looking dipictions of sea serpents? The oar fish is a likely candidate for sea serpent sightings.