The eye was of the size of the Seal's

In September 1808, a local man was fishing off the coast of Orkney when he spotted a bunch of birds feasting on an animal corpse on the rocks. He saw what looked like a giant sea serpent, but couldn't get close enough to be sure.

Ten days later, a gale washed the corpse onto the Stronsay beach, where it was closely examined and measured by local men. The beast quickly gained notoriety--but it also quickly decomposed. The four men who examined it closely were taken to Kirkwall (Orkney's main city) to give sworn testimony to the magistrate about what they'd seen. The 1860 Naturalist's Library sums up the testimony as follows:

It measured fifty-six feet in length and twelve in circumference. The head was small, not being a foot in length, from the snout to the first vertebre; the neck was slender, extending to the length of fifteen feet. All the accounts agree in assigning it blow holes, though they differ as to their precise situation. On the shoulders something like a bristly mane commenced, which extended to near the extremity of the tail. It had three pairs of fins or paws connected with the body; the anterior were the largest, measuring more than four feet in length, and their extremities were some what like toes, partially webbed.[...]The skin was smooth, without scales, and of a greyish colour; and the flesh appeared like coarse ill-coloured beef. The eye was of the size of the Seal's; the throat was too narrow to admit the hand.

I just want to say I respect whichever of the observers (described by the Library as "respectable individuals," in case we were worried) stuck his hand down this thing's throat.

If you're interested, the full eyewitness testimony can be accessed from Yvonne Beale's dead geocities page via the Wayback Machine, here. And it is fascinating! The bristles glowed in the dark when wet! George Sherar, tacksman (meaning, I believe, that he paid a fixed annual sum to the government in exchange for the right to collect certain traditional taxes in a designated area of Orkney, whatever he collected over the amount he paid in "rent" being his own profit), clearly had a scientific mind. He did several experiments, including putting a small piece of the animal in his lamp to see how it burned: "it neither flamed nor melted, but burned away like a gristly substance." He says "that he was the more attentive to its shape, dimensions and figure, in order to be able to give an accurate account of it to any travellers that might come to Rothiesholm," and I hope he dined out on the story for the rest of his life!

The November 1808 meeting of a Natural History Society in Edinburgh gave the creature the Latin name Halsydrus Pontoppidani ("Pontoppidan's Water Snake of the Sea") in honor of an 18th century Norwegian bishop with a sea-serpent hobby. But a leading ichthyologist of the day, Sir Everard Home, a member of the London Royal Society, examined the remains and believed the creature to simply be a basking shark, a large fish common in the Orkney waters. Several scientists since then have supported his conclusions.

Basking shark.

"How the heck could that look like that sketch up there?" you may be wondering. The answer is that basking sharks take on a "pseudo plesiosaur" appearance while decomposing! Their jaw drops off, giving the appearance of a long neck and tiny head, and then,

[...]as only the upper half of the animal's tail fin carries the spine, the lower half rots away and provides a convincing serpentine tail. When the dorsal fin begins to decompose, the remaining rays can have the appearance of a hairlike mane. The monster's six legs can simply be explained away as the remains of the shark's lower fins.

(from this site.) 

The thing is, the largest recorded basking shark is only 40 feet in the length, and the Stronsay Beast was carefully measured by more than one person as being in the 55 foot range (and part of the tail seemed to be missing, so it might have been longer!). Sir Everard Home got around this issue by insisting that the three eyewitnesses who measured the corpse were not educated men, and must have measured wrong. Never mind that one of them was a carpenter! (The other two were farmers, but since farming in Orkney at the time involved a complicated division-of-land system that relied on a lot of measuring, I'm guessing they knew what they were doing.) He blithely announced that the creature was really only about 35 feet long.

The Royal Museum of Scotland in Edinburgh has the vertebrae and bristles (although I don't remember seeing them when I was there, which makes me very sad!), the skull and a "paw" were sent down to London and destroyed in the Blitz, and some other pieces ended up in the hands of Lord Byron. I've googled and googled and I can't figure out how! He doesn't seem to have been in Northern Scotland at the time. Anyone? Those pieces are still around and now in the John Murray Collection at the National Library of Scotland.

An Orcadian scientist named Yvonne Beale (who has a degree in evolutionary, environmental and biomedical genetics from St. Andrews and a doctorate in DNA damage repair from Edinburgh's pathology department) has said she can use those pieces to determine the creature's species. But her current website is still under construction, and the archived geocities page from 2008 doesn't come to a definitive conclusion.  She questions the "basking shark" theory on the ground that basking sharks only resemble plesiosaurs when pretty decayed, and nothing about the eyewitness testimony suggests that the Beast was too badly rotted. But she does say that since the Beast is cartilagenous, it couldn't be a whale or real plesiosaur, and must be a type of shark. Every other story I can find just says she's going to make an announcement, and they're all dated three or four years ago. I can't even find confirmation she was ever given permission to take samples from the remains in the National Library's possession.

And so the cryptozoology debate continues...

If you were going to use this story in a romance, who would your hero and heroine be? And seriously, how do you think Byron got those Beast pieces?