Merhorse, from 13th century Manuscript De Natura Rerum (Things of Nature) by Saint Albertus Magnus

Merhorse (Hippocampus), from 13th century Manuscript De Natura Rerum (Things of Nature) by Saint Albertus Magnus

The actual name in the text is "The Marine Horse" or the "Horse of the Sea"
Albertus Magnus was an outstanding scholar in the 1200s who was reviving interest in Classical sciences and the Scientific method. He quoted extensively from Aristotle and no doubt was quoting extensively from Classical sources.
https://en.wikipedia.org/wiki/Albertus_Magnus
https://en.wikipedia.org/wiki/Hippocampus_(mythology)

There seems no direct connection between the Sea Horse of Albertus Magnus and the Sea Horse-Sea Serpent of Olaus Magnus

Swedish ecclesiastic and writer Olaus Magnus's Carta marina, many marine monsters of varied form, including an immense sea serpent, appear. Moreover, in his 1555 work History of the Northern Peoples, Magnus gives the following description of a Norwegian sea serpent:

Those who sail up along the coast of Norway to trade or to fish, all tell the remarkable story of how a serpent of fearsome size, 200 feet long and 20 feet wide, resides in rifts and caves outside Bergen. On bright summer nights this serpent leaves the caves to eat calves, lambs and pigs, or it fares out to the sea and feeds on sea nettles(jellyfish), crabs and similar marine animals. It has ell-long hair hanging from its neck, sharp black scales and flaming red eyes. It attacks vessels, grabs and swallows people, as it lifts itself up like a column from the water.

 https://en.wikipedia.org/wiki/Sea_serpent

It should be noted that long hair of only a few feet thick would be miniscule growth on a creature 200 feet long and 20 feet wide. This description seems to combine the basic features of both the Long-necked Sea Serpent and the Whale-eater, which in the latter case is not that big of an exaggeration. The black colouration is due to its body being I the shadows or on the shadow side. Presumably all of the food animals suggested for it were mistakes and pure guesses rather than based on any direct observations,

As to what the classical Hippocampus was, that is difficult to say. Depictions do integrate into depictions of Capricorns and to other creatures actually called Sea Serpents, but then again on the other hand to creatures called whales and dolphins. For the time being I shall continue to say they are all depictions of the Long-Necked sea serpent: but if good evidence be forthcoming that there was a moderately large seal in the Mediterranean, bigger than the monk seal, that might have been the original (Especially since most depictions make them out to be well less than 20 feet long: one variation is also called the sea leopard, much the same idea as in Leopard seal. These creatures in general seem to be analogous to the Indian Makaras)