I was just looking at my copy of Roy Mackal's Monsters of Loch Ness, in particular at his reconstructions of his theoretical long-necked newt candidate. It struck me that the overall proportions of his creature corresponded fairly well with those of the known giant otter of South America, and that could be a significant component out of the sightings averaged out into his reconstruction. Conversely, if you left the tail off, the front part could be a good representation of the Hoy-type long-necked sea lion with the adustment made for the given measurements not matching the drawing. The neck might well be somewhat longer than Mackal's reconstruction and evidently he was assuming a high back fin on the creature. The overall length of the Long-necked seal and the master-otter could be similar. and when I checked my redrawn versions the heads of both were in good proportion to the relative proportions of each. Checking this against the statistical average for long-necked sea serpents, the length of the neck is about equivalent to the entire length of the other creatures but the neck was very much thinner and the head very much smaller in absolute measurements according to the reports. A length of over ten feet with a thickness of one foot is typical for the long-necker reports, although only the first half of that might be visible: the head is just about absolutely the same size as the giant otter's head, half in all dimensions from the long-necked sea lion. The shape of the head is also different: it is flatter on top with a smaller brain case relatively, and usually compared to a snake's head. This would also correspond to Mackal's reconstruction reversing the proportions of the head-neck and tail relative to the length: Mackal admitted to doing exactly that with several of the reports.
The giant eel reconstruction also in Mackal's book is probably misleading because it does not match verifiable giant eel reports. The giant eels seen in freshwater are about the same length as given for the Plesiosaurian long-necks (both average sizes drastically less than the corresponding saltwater report average dimensions) which is usually given as 20-30; more rarely 40 feet long. At this length the eel types are markedly different in shape, being a more uniform over all width per length, and the forepart is very much thicker than the corresponding long-necker's periscope. The head is easily the biggest out of all of these types, and probably 20 times the long-necker's head for the same length (The long-necked sea lion has a much larger head and a much shorter length over all than the typical long-necked sea serpent. At perhaps 15 feet long, not counting the hind flippers, its head is probably ten times the size of the 30-foot-long Plesiosaurian long-necker, by the statistics) A 30-foot-long giant eel can typically be a yard thick, and its head easily 4 feet wide by 5 feet long. It is described as a truly frightening sight by witnesses close up.
And then again, a great many reports of heads like horses, cows, sheep and goats can most often be put down to sightings of moose heads outside of the antler-bearing season. The head of the long-necked sea lion (at probably 15 feet long average, about walrus-length) is also said to be about the same absolute size as a horse's or cow's head but shaped very differently: when the same is said of the long-necked sea serpent types, they are otherwise stated to be at a significantly larger size over all, in the range of 40 to 50 feet long, which in turn agrees with Oudemans's tables counting a shortened tail.
Best Wishes, Dale D.
P.S. The statistical extractions are all my own estimates based on my own assessment of the reports. This has so far gone unpublished despite several firm offers to publish the data made to me in the past. They all fell through.