There have been suggestions that the nessie type creatures migrate between the lochs using the Caledonian Canal. This a man made 62 mile canal which connects the Scottish east coast at Inverness with the west coast near Fort William. However any large creatures would have been seen and there appears to be no reports of sightings.
.The first reported sighting of the Loch Arkaig creature was in 1857:
Excerpt from the Diary of Lord Malmesbury October 3, 1857:
October 3rd. This morning my stalker and his boy gave me an account of a mysterious creature, which they say exists in Loch Arkaig, and which they call the lake-horse. It is the same animal of which one has occasionally read accounts in the newspapers of having been seen in Highland lochs, and on the existence of which in Loch Assynt the late Lord Ellesmere wrote an interesting article, but the story has always been looked upon as fabulous. I am now, however, persuaded of its truth.
My stalker, John Stuart, at Achnacarry, has seen it twice, and both times at sunrise in summer on a bright sunny day, when there was no ripple on the water. The creature was basking on the surface. He saw only the head and hind quarters, proving its back was hollow, which is not the shape of any fish or seal.
Its head resembled that of a horse. It was also seen once by his three little children, who were walking together along the beach. It was then motionless, about 30 yards from the shore, and apparently asleep, and at first they took it for a rock, but when they got near it, it moved its head, and they were so frightened that they ran home, arriving in a state of greatest terror. There was no mistaking their manner when they related this story, and they offered to make an affidavit before the magistrate.
The only more recent sighting I could find is reported in Tim Dinsdale’s book “The Leviathans” on page 240.
( you can find this online to read at google books). Tim wrote that he met two men who had been commandos in WW2 based in the Loch Arkaig area. They said they had been out on the loch in an inflatable boat when an upheaval in the water almost swamped them. They said it must have been a very large fish or the monster.
It would be thought that there would have been more sightings over the years like at Loch Ness . There have been sightings at nearby Loch Lochy of a creature so maybe it moved home? There is also another story attached to Loch Arkaig of Jacobite gold. The monster story could have been put about to keep treasure hunters away.
The story of treasure /gold at Loch Arkaig