Merpersons

[-JKP-]

The settings for these 'mermaids' is not very orthodox.
By their very nature, mermaids were used to illustrated far away places in the deep sea.
I would suggest that identifying these objects as mermaids would set the location as somewhere far away and nautical. And in anycase, the setting seems far too ripine for a mermaid.
So what are they? I have some thoughts but nothing ready for publishing at the moment. But I am concentrating on something continental, not nautical, in keeping with the general feel of the pages.
The Hunterian image, by the way, is not a very prominent image. It extends out of a initial on a page, and isn't very large at all. I suggest the only reason the modern curators fixated on it was because it is a rare example of an extension to a decorated initial, usually such decorations are confined to the inside of the letter.
Notwithstanding that, I admit there is a comparison to be made, and well done for spotting it.

Mermaid-like creatures with two tails are based on depictions of Pisces with more stars (a woman and two fish) and typically have two tails—some with the earth goddess holding the fish and some with the earth goddess with fish legs. The reason the legs always bend up back toward the hands is because originally the fish were fish, not legs, but are perceived as legs and became iconographically represented that way (similarly unicorns, which were originally split-hoof animals, probably goats, have become horses rather than goats according to popular taste).

Mermaid-like creatures illustrated in the sea usually depict sirens, mermaids, and variations on the Noah Jonah story.

When they are fresh-water "mermaids" they are either water creatures (along with fish, crayfish, turtles, etc.) or they are melusines which were originally Pagan fresh-water goddesses/sprites/faeries but which later became associated with specific legends as Paganism declined and Christianity spread. They are found in Frankish/Gallic texts as well as English in the 14th and 15th centuries, first as textual stories, later as illustrated legends.