I guess it's another Monster Monday.
"Jenny Haniver" is a term with any number of possible etymological fathers but no clear origin. It refers to folk-art creations of sailors in the medium of dried skate (the fish), purporting to be preserved carcasses of fantastical monsters.
humanoid creatures under the sea with pointed and tonsured heads ...
Um ... okay, well maybe it does.
JENNY HANIVER: Size S, numbers appearing 2d10, HD 1+1, AC 8 , MV 6 flying/6 swimming, AT d3 bite, Intelligence: animal.
This is a curious flying marine predator of far-away climes, with leathery skin, about the size of a small cat. They travel in flocks and are known to leap on board a ship en masse to harass and bite the crew. For this eventuality, wise captains keep stocks of netting on board when sailing haniverous waters. They mummify easily and some sailors make a living from selling them.
SEA MONK: Size M, numbers appearing d12 or d12x3 in lair, HD 3+1, AC 5 , MV 6 swimming, AT d3/d3 nonlethal flippers, spell use as 3rd level cleric, Intelligence: average.
These gregarious creatures collect in small communities to farm seaweed, raise sea snails, and serve various undersea deities, with some "abbeys" being lawful and some more sinister. Lairs are often guarded by 1d6 monk seals.
SEA BISHOP: Size M, numbers appearing d3, HD 7+1, AC 3 , MV 6 swimming, AT d6 headbutt nonlethal, spell use as 7th level cleric, DF ink cloud. Intelligence: high
Be they pious or debauched, these creatures have a strong connection to the kind of deities worshipped by sea monks, but prefer to lead and advise others rather than group together. They can be found in communities of sea monks, mermen, tritons, locathah, or sahuagin. In danger they can spew a 20' radius ink cloud in the water to confuse enemies and make good their escape.
* As far as I can tell Steenstrup and "Mr. Sluperius" were real guys and not fake tome authors from an off-brand Mythos tale.