Thomas Finley said when he made this illustration:
"This painting captures an elusive Merhorse both above and below the waves and gives a special over all view of the size of the creature. This is a special request for the Bizarre Zoology Blog Series."
I then objected that Bernard Heuvelmans had made several specifications about the Merhorse, including that it was supposed to have enormous eyes. After some discussion (and submissions of competing artworks by both of us), Thomas Finley developed what we thought was a good and authoritative Merhorse ex Heuvelmans.
However it seems Jay Cooney had originally intended this painting to represent a more generic giant pinniped which could account for reports contained in the Merhorse category. We can perhaps equate this to Peter Costello's version of the Longnecked Seal theory more than Heuvelmans' version per se (There is a good reason for saying so and we shall be getting to it directly)
biomarginalia (Presumably Cameron McCormick) writes:
[Regarding]Bernard Heuvelmans’ “Long-Necked" sea serpents: the Hoy “sea serpent" is fascinating, although rather than evidence for a long-necked species of pinniped, I wonder if it was an encounter with a very very wayward (and exaggerated) eared seal. As for the others, it is within the realm of possibility for mirages to create the impression of a long-necked creature (I’ll have to track down that diagram). Confusingly, it is not explained why the Tonny and Orme’s Head encounters are classified unambiguously into this category (and not, say as Super Otters or Many-Humped); amazingly the Orme’s Head sighting was published in the rather improbable venue of Nature. It may be a bit more difficult to publish a sea serpent report there today. [These drawings were published in the recent blog on Longnecked reports in general-DD]
Bernard Heuvelmans' version of the Long-necked giant Sea lion, his "Megalotaria"
In his account, J. Mackintosh Bell had stated that the neck above water was as thick as an elephant's foreleg and all rough looking. An elephant's foreleg is a much thicker object than is shown in the original drawing and an attempt to make an appropriate corresponding thickness of neck is what is shown in the above paste-up.
There is one good Scientific account and illustration of a longnecked seal, made in the late 1700s and evidently making reference to a creature sighted around the British isles and not actually seen by the artist himself. This is nonetheless the only really substantial documentation of the allegation. In this case, a young male was said to measure 7 1/2 feet long and the projected size of an adult male would then be between 20 and 25 feet long (Pehaps 30 feet long counting the outstretched rear flippers)
(The adult male sea lion is about three times as long as the pup, a statement affirmed by Heuvelmans)
This information was provided by Darren Naish:
James Parson wrote a paper in 1751 in which he described five “species” of Phoca, among them he mentioned a Dr. Grew’s “long neck’d seal” from an unknown locality. This peculiar seal was actually part of the Royal Society’s Museum, and as such it was included in a Catalog published in 1681, where it was described as follows:From his nose end to his fore-feet, and from thence to his tail, are of the same measure 
Heuvelmans' composite is not a good match for the Long-necked Sea lion:
As I had said it before, I prefer to refer to the Long necked Sea Serpent reports collected by Heuvelmans as "Megalotaria longicollis" to represent two types, the majority being the Longneck longicollis but this series as typified by the 1919 Hoy Sea Serpent retaining the genus name Megalotaria (Big Sea lion). I would have preferred to retain the name Megophias preferred by Oudemans for the Longicollis creature, but I have heard some very persuasive arguments why that name should not be maintained. Incidentally the colloquial term "Long-neck" as a reference to the long-necked plesiosaurs (and as distinguished from the short-necked plesiosaurs) runs back as far as the middle 1800s (the middle of the 19th century, and old enough for the term to be understood since before the American Civil War. References to Oudemans' composite creature as a Long-neck or a Long-necked seal were not merely descriptive, they were making a direct acknowledgement that his Megophias looked like a long necked Plesiosaur) Thus the uses for Long Necked seal and Long Necked Plesiosaur are almost equally as old as each other: it cannot be objected that the term is a modern one or that Heuvelmans made it up.