Saint Columba (565)
The earliest known report occurred in the Life of St. Columba by Adamnan, written around the 7th century. It describes how in 565 Columba saved the life of a Pict, who was being supposedly attacked by the monster. Adamnan describes the event as follows:
"...(He) raised his holy hand, while all the rest, brethren as well as strangers, were stupefied with terror, and, invoking the name of God, formed the saving sign of the cross in the air, and commanded the ferocious monster, saying, "Thou shalt go no further, nor touch the man; go back with all speed." Then at the voice of the saint, the monster was terrified, and fled more quickly than if it had been pulled back with ropes, though it had just got so near to Lugne, as he swam, that there was not more than the length of a spear-staff between the man and the beast. Then the brethren seeing that the monster had gone back, and that their comrade Lugne returned to them in the boat safe and sound, were struck with admiration, and gave glory to God in the blessed man. And even the barbarous heathens, who were present, were forced by the greatness of this miracle, which they had seen, to magnify the God of the Christians".
Skeptics question the reliability of the Life of St. Columba as evidence for the Loch Ness Monster's existence, noting that the book describes implausible events, such as an incident when Columba slays a wild boar by the power of his voice alone. They argue that the monster encounter is said to have occurred on the River Ness, not in the Loch, and that Adamnan reports Columba encountering and conquering assorted "monsters" at various locations in Scotland, throughout his life. Moreover, sceptics assert that there are no other accounts of the Loch Ness monster attacking anyone, as the creature is normally portrayed as shy. In fact, biographies of the early saints were often embellished or invented for purposes of religious persuasion rather than historical record.
Source & References: