There are many examples in human folklore and art which have descriptions and illustrations of dinosaurs. Mokele Mbembe, the stone carvings of an abby on Mount Gergeti in Georgia (the Eurasian one, not the American one), “thunder birds” (pterosauria) in the North American southwest, the semitic legends of Leviathan and Behemoth. Although this is more of an anecdote, I've read that pre-industrial Chinese folk identified dinosaur bones as dragon bones when they were first uncovered there.

It is possible that the original Loch Ness Monster was simply a form of kelpie, not actually a plesiosaur. I don't know enough about plesiosaur biomechanics to say whether it would be possible for a colony of them, or even for one long-lived individual, to have been living in the Loch — of course, I don't think many people alive today could say otherwise.