Columba

Columba or Colmcille (7 December 521 – 9 June 597) was an Irish abbot and missionary evangelist credited with spreading Christianity in Scotland. There are several medieval accounts and local legends surrounding St. Columba. According to some authors, Columba encountered the Loch Ness Monster during the 6th century, however, the water demon described in Adomnán's The Life of St. Columba, Founder of Hy has several notable differences from the Loch Ness Monster.

Meeting King Bridei and the Wizard Broichan
In the 6th century, Columba visited the pagan king of the Picts, Bridei, at his fortress by Loch Ness. Broichan, a pagan wizard, was close to the king and that he also possessed an Irish slave-girl who had possibly been a war captive. When Columba met Broichan and Bridei at the king's fortress near the River Ness, Columba demanded that the girl be released but the druid refused.

Columba then went down to the river and told his companions that an angel at that moment had struck Broichan, and his cup had fallen from his hand and he was deathly ill. Moments later two messengers from King Bridei came to inform Columba of these events as they happened, and Columba gave his own messengers a pebble from the River Ness and told them that if Broichan agreed to release the slave-girl, then they were instructed to place the pebble in a cup of water and make him drink it, but if Broichan refused, then he would die immediately. They then went back to the fortress and Broichan agreed to release the girl, and so they placed the pebble in a cup of water, and it miraculously floated on top. Broichan drank from it and was healed.

Monster in Loch Ness
In Adomnán's the Life of St. Columba, Founder of Hy, the creature is referred to as an unholy water beast that was exorcised by St. Columba. "ON another occasion also, when the blessed man was living for some days in the province of the Picts, he was obliged to cross the river Nesa (the Ness); and when he reached the bank of the river, he saw some of the inhabitants burying an unfortunate man, who, according to the account of those who were burying him, was a short time before seized, as he was swimming, and bitten most severely by a monster that lived in the water; his wretched body was, though too late, taken out with a hook, by those who came to his assistance in a boat. The blessed man, on hearing this, was so far from being dismayed, that he directed one of his companions to swim over and row across the coble that was moored at the farther bank. And Lugne Mocumin hearing the command of the excellent man, obeyed without the least delay, taking off all his clothes, except his tunic, and leaping into the water. But the monster, which, so far from being satiated, was only roused for more prey, was lying at the bottom of the stream, and when it felt the water disturbed above by the man swimming, suddenly rushed out, and, giving an awful roar, darted after him, with its mouth wide open, as the man swam in the middle of the stream. Then the blessed man observing this, 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.""