The men took the body attacked by Niseag out of River Ness by hook and proceeded to ask another man to row a moored cable out to the boat. While this man was swimming, a beast came after him, but then fled when Saint Columba spoke and presented the sign of the cross to it. An estimated size or further details were not provided. We do know, based on a later mention in the accounts, the water creature likely wasn't a whale of some sort. In those details, Saint Columba performs another miracle by preventing a whale from attacking men setting to sea.
There are several known legends of water dwelling monsters in Scottish, Irish and Pictish history. Saint Columba's experience may boil down to how ancient cultures in the area described animals at the time, even if those creatures seemed to only exist for mythological purposes by modern science. Two such creatures exist in Pictish folklore evident by symbols found in their ancient stone carvings. One resembles the shape of a dolphin, dragon, horse or elephant and is popularized today as the Pictish Beast. The other is the Water-horse, or highland bull, claimed to haunt pools and lochs to lure children to their deaths. In both cases we find distinct features which could match up with Columba's encounter to suggest it might be a play on folklore for the benefit of an Evangelistic journey, but nothing quite specific enough for plausible identification of a past Nessie relative. Both Irish and Scottish lore talk of similar beasts, namely the River Wolf, Water Hound, Irish Crocodile, and Dobhar-Chu. Although these animals are traditional described as half-dog half-fish, they eerily fit along with Columba and some Nessie descriptions.
Modern science and archaeological record might have a logical answer which unfortunately may not be a sole surviving Plesiosaur as some hoped it would be. Characteristics of Nessie sightings tend to fit Plesiosuchus, a dinosaur who disappears from fossil record roughly around 4 million years ago. Characteristics of water beast sightings such as the Dobhar-Chu tend to fit another dinosaur called Dakosaurus, a prehistoric whale alive during the same era as Plesiosuchus.
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