(Sorry this is so late. Being sick and dealing with allergies at the same time BLOWS!)
So, it's not really 31 Days of Monsters at this point, but I would like to get through 31 monsters total at least, and there are some of the really famous cryptids that I was leaving until last that I didn't get through. The Loch Ness Monster was one I had originally planned to do as a kind of 'Grand Finale'. Well, we can all see now how that turned out. Anyway, I'm not going to do these one after the other, so expect more like this to be somewhat spaced out, but I am going to finish this little series. Eventually.
Size: Estimates from video, photographs and sonar recordings very widely, from as small as 5 feet to as long as 46 feet.
Appearance: Generally a large, oval-shaped body, a long serpentine neck, and a small head. It has drawn comparisons to the prehistoric sea reptile the Plesiosaur in nearly every description.
Threat: Low. While a creature of Nessie's size could certainly cause harm to and possibly even eat people, there have never been any reports of attacks. Though, with water temperatures ranging between 42 and 58 degrees, it's not a common swimming spot for tourists.
The Loch Ness Monsters is one of the oldest cryptids known, and one of the most interesting in that it seems to have kept up with technology without giving itself away. There have been dozens of searches of the Loch, including intensive sonar scans. Many have indeed turned up shapes of large animals moving deep under the water which cannot be identified. Of course without a body or a crystal clear video, there is no definitive proof on the animal; but as long as sightings continue, both visually and on sonar screens, the mystery will continue.
Many suggestions have been made for what the creature could be. All have strong points, as well as weaknesses. One of the best ideas put forth was an oversized Conger Eel. Color would match and size would be a good fit if one could grow large enough. The problem with that answer, though, is that Conger Eels are saltwater fish, and the loch is freshwater. Their long, snake-like bodies also don't come close to the bulkier form often reported.
Even to this day, expeditions and searches carry on on the Loch. Jeremy Wade did an episode of his show River Monsters where he plumbed the depths of the loch to see what was in it, then followed the trail of the stories through history and made a guess that the massive and rare Greenland shark could be the culprit. Again though, this answer has holes in that the Greenland shark is a salt-water species. It also doesn't match the long neck and head so often described.
Of course, there have also been several hoaxes, made by people looking for a little publicity or just trying to have fun at the expense of those who think there is really something there. In fact, it was even brought out several years ago that the original Surgeon's photograph (The first one up top) is a hoax, done for no real reason than to stir up the local press. Most recently, a picture reported to be on Apple Maps supposedly shows the creature swimming near the surface of the loch. That one has since been shown to be simple a photo of a boat where the offending vehicle has been photoshopped out.
Still, whether there is an ancient Plesiosaur swimming around in Scotland, or not. It's hard to deny that there does seem to be something in the lake that we are not familiar with. Something large, swimming at depth, occasionally captured on sonar, and more rarely, seen, and even photographed. Believing in it or not is entirely up to you. At the moment, the evidence swings both ways.
Still, if you're out in Scotland, I would recommend trying not to fall out of your boat. Just to be on the safe side.