How Did the Idea of the Loch Ness Monster Start?

In this video:

Loch Ness is a long, narrow lake southwest of Inverness in the Scottish Highlands. It is the second largest loch in Scotland by surface area and the largest by volume. It is also the second deepest loch, at an astounding 755 feet (230 meters) at its deepest point. The water is dark and murky due to the high levels of peat in the surrounding soil. The breadth, depth, and low visibility of this particular loch make a prime environment for someone’s imagination to run wild, particularly when other lake monsters were plentiful in Scottish legends.