This is a mermaid, or as the Gemarah calls it (Bechoros 8A) a dolfinin. What's that you say? Dolfinin, sounds a lot like Dolphin? Hey. GMTA. I though so, too. And my first impression was bolstered by the breisa's description of the dolfinin, a description which sounds an awful lot like Flipper: "They reproduce like people." Even better, per the dictionary, our English word "dolphin" comes from the Greek delphis (gen. delphinos) "related to delphys "womb," probably via notion of the animal bearing live young."
So why am I translating "dolfinin" as "mermaid?" Simple. DovBear has emunat chachamim (lit: faith in the Sages) and Rashi said they were mermaids. Though you are welcome to argue with Rashi over on the menuvel blogs, here we have blind, yet complete, faith in Rashi and in his credentials as a marine biologist.
This is how Rashi treats the word dolfinin. First, he emmends the text so that it reads "reproduce *from* humans" instead of *like* humans, which, as you recall, is the sort of non-Orthodox move that nearly got Ruven Malter expelled from yeshiva in The Promise.
Then, Rashi writes: "there are fish in the sea whose form is half-human and half-fish, [they are called] sereine in Old French." Those of you who read the Odyssey, like Rashi no doubt, will realize that here Rashi is referring to the siren, a mystical creature with the tail of a fish in place of legs.
So, as they say in the shteeble: Case closed.
(Ha! Only kidding. The case is not closed, not by a long shot. See the comments.)