Did the Loch Ness monster ever exist?

My guess is that it may have been a large sea creature, thought to be extinct that somehow made it's way into the Loch Ness for a period of time before going back to the sea and thus only the original sightings hold any efficacy.

The earliest written reference to a monster in Loch Ness is a 7th-century biography of Saint Columba, the Irish missionary who introduced Christianity to Scotland.

In 565 A.D., according to the biographer, St. Columba was on his way to visit the king of the northern Picts near Inverness when he stopped at Loch Ness to confront a beast that had been killing people in the lake.
Seeing a large beast about to attack another man, St. Columba intervened, invoking the name of God and commanding the creature to “go back with all speed.” The monster retreated and never harmed another man.

So why would a saint lie to people about it?!!