This legend has certainly existed for many centuries in Scottish folklore.
However, the story of the monster was not very well-known in England for one simple reason: Loch Ness is a very long way from the rest of Britain. Until the age of the railway, very few people ever went to the Highlands of Scotland, except soldiers or officials from the cities of the Scottish Lowlands. No-one else had any reason to go there: the North of Scotland was wild and desolate, wet and generally cold, and inhabited more by sheep than by people.
The most famous photo of all was taken in 1934 by a London surgeon; it seems to show a long neck and a small head sticking up out of the water. "Nessie" - if the photo is real - looks something like a dinosaur.
In 1987, some people used sonar equipment to try to discover Nessie.... but they found.... nothing. So no-one has proved that the Loch Ness monster exists; but no-one can prove that it does not exist. It's a great story.