(It appears in the Life of St. Columba by Adomnán, written in the 7th century. According to Adomnán, the Irish monk Saint Columba was staying in the land of the Picts (Scotland) with his companions when he came across the locals burying a man by the River Ness. They explained that the man had been swimming when he was attacked by a “water beast” that had mauled him and dragged him under. They tried to rescue him, but were able only to drag up his corpse. Hearing this, Columba stunned the Picts by sending his follower Luigne moccu Min to swim across the river. The beast came after him, but Columba made the sign of the cross and commanded: “Go no further. Do not touch the man. Go back at once.” The beast immediately halted as if it had been “pulled back with ropes” and fled in terror, and both Columba’s men and the pagan Picts praised God for the miracle.
The earliest report of the Loch Ness monster appeared in the 7th century.
Of course I couldn’t help but notice that he sent his assistant across the river.)