Hm. It seems I lied to someone recently. Or, at least, told them something that wasn't true. I get a lot of questions and I forgot who asked the question so this is for you, person who asked about mermaids. Turns out: there are mermaids in Hellenic mythology--at least locally so.

I recently came across this tiny snippet in Diodorus Siculus' "The Library of History":

"Now there is in Syria a city known as Ascalon, and not far from it a large and deep lake, full of fish. On its shore is a precinct of a famous goddess whom the Syrians call Derceto; and this goddess has the head of a woman but all the rest of her body is that of a fish, the reason being something like this. 3 The story as given by the most learned of the inhabitants of the region is as follows: Aphrodite, being offended with this goddess, inspired in her a violent passion for a certain handsome youth among her votaries; and Derceto gave herself to the Syrian and bore a daughter, but then, filled with shame of her sinful deed, she killed the youth and exposed the child in a rocky desert region, while as for herself, from shame and grief she threw herself into the lake and was changed as to the form of her body into a fish; and it is for this reason that the Syrians to this day abstain from this animal and honour their fish as gods." [4.2]

Well then! Derceto was new for me (trust me, I do not know everything about ancient Hellenic mythology, especially not local deities). It seems to be a different (local) name for Atargatis, the chief Goddess of northern Syria in Classical Antiquity. She's a protective Goddess as well as a fertility Goddess and her sanctuaries had ponds close by. The priests were the only ones allowed to take care of the fish in the ponds, and if I remember well, they were also the only ones allowed to touch them. Atargatis' worship did travel into ancient Hellas, but I am truly not sure how wide-spread Her worship was. I do know that the mermaid myth was local to Ascalon, not even extending to the other shrines of Atargatis.

So, there: mermaids in ancient Hellenic mythology. It's slim, but it's there. Sorry I can't let you know directly, person!