Lusus Naturae. A freak, mutant or monster. A deformed or strangely marked creature.
Never heard of this word before reading this article and when I read it, for a second I thought all of my sea-monster article collecting would come to nix, with ‘sea monster’ being slang for a known creature. I was wrong. It was a word used by the captain to make it seem like he knew what he was talking about. The children thought it was seaweed, the carpenter thought it was seaweed but the captain said it was a sea monster, so sea monster it was. I love that the captain describes it as ‘the sea serpent in one of its early stages of development’. He obviously knows enough about sea monsters to recognize an adult sea serpent, it’s just the fact that this one was a juvenile that threw him off a quick identification.
I think that even a landlubber like me would be able to tell the difference between a sunfish and something described as a ‘serpent’ though. I have never come across a giant sunfish anywhere but on the telly, but I have noticed that they are far from serpentine. Blobby, perhaps. Not serpenty.
(Kilmore Free Press 23rd March 1882)
Buried words and Bushwa. 
I have seen a sunfish in person (uh, in fission?) and it was, as you say, the least serpenty thing on the planet. It is like a giant round coaster and the one we saw just swam in sad circles. It was genuinely pretty freaky, but serpentine? Perhaps he had drunk all the rum.