0657 (pre): St. Cummein’s Fields
St. Cummein [?-669], seventh abbot of Hy on the Scottish island Iona (presided 657-669), was said to have previously founded a church at the western end of Loch Ness, in about the area today called Fort Augustus. He needed to clear some land by plow, but the work proved so difficult that his own monks soon rebelled and refused to do any more of it. Cummein, unwilling to force the monks to do the work, retired to his cell to pray and think on the matter.
The following day, a group of red deer came down from the nearby mountains to St. Cummein, and allowed themselves to be harnessed to do the work. As the monks watched with a mix of amazement and shame, the stags ploughed the fields steadily all through the day, returning to the mountains that night with Cummein's blessing.
After nightfall, two of the shamed monks arose and went to the field, fully intending to work the plough until dawn; but the plough was no longer where it had been left standing, next to the waters of Loch Ness. A quick investigation located the missing plow... it was being used by a single great black stallion to plow the field, in place of the team of deer! The beast was the water-horse of Loch Ness itself; and when the dawn rays first appeared and the job of plowing the fields was completed, the beast returned to the lake, taking with it the first monk who had refused to help plow the fields.
Unreliable & Under Investigation
How sad is it to mark a legend as 'Unreliable'? But unreliable it is, as I've seen no sources for it much older that 1969... which is not old enough if it is to be claimed as a genuine legend related to St. Cummein! Cummein was probably a chosen historic figure for this tale because he was associated with an area near Loch Ness and, as a saint, presumably any tale told about him would be considered true. It should also be noted that St. Cummein wrote and published under the name of Adamnan The Life of St. Columba: Founder of Hy, which is the very same volume that the legend of St. Columba meeting a beast in the River Ness originates from... which is considered by many the first mention of an encounter with a monster in Loch Ness.
The tale of Cummein and his fields seems to first appear in Frederick William Holiday's 1969 book, The Great Orm of Loch Ness. Holiday may have created it, because I sure can't find any earlier historic source for the story yet!