Mental Floss has a list of 7 explanations for the Loch Ness Monster. That is, 7 things that people might be seeing and misinterpreting as Nessie. The list is: lake sturgeons, surfacing trees, indigenous eels, mountainous reflections, bird wakes, bubbles produced by seismic activity, and swimming elephants.
Over the years, there have been quite a few more explanations than this offered to account for Nessie sightings. And some of the explanations are definitely strange. One of these days I'll get around to producing a full list of them.