The Talmud has a passage that discusses the mating habits of various animals and fish. Why that is important to the Talmud, we can discuss another time.
One marine species is mentioned that reproduces the same way as humans, and the Talmud refers to this species by its Latin name. The rabbis of the Talmud described this species as "sea people."
This mysterious reference to sea people has sparked the imagination of many students of the Talmud. In fact, the great medieval scholar Rashi defined them as being half human, half fish, and translated them into his native French as sirene, meaning mermaids.
But perhaps we would translate it differently. Because the original Latin name quoted in the Talmud uses for these humanlike sea creatures is Dolphinin.
Indeed dolphins resemble humans in some of their mating behaviours. The Talmud was correct to say that they reproduce like humans. But Rashi's text of the Talmud read differently. It said that they reproduce from humans, not like humans. In Hebrew the difference between 'like humans' and 'from humans' is but one letter:מבני אדםorכבני אדם. They appear very similar, but their meanings are far apart.
If this is indeed a misprint, it would explain why Rashi understood that the Talmud is here confirming the existence of mermaids - a perfectly plausible belief in medieval France - while in fact the Talmud is acknowledging the unique affinity between dolphins and humans.
But don't be too disappointed. Even if we don't believe in mermaids, we do believe that people are sometimes half animal. Our life challenge is to elevate our lower half to serve the upper half.