There is no such thing as a mermaid or siren which can procreate with man. This is a scientific fact and therefore we can either say that Chazal were quoting popular science of their time which was mistaken or that we can understand their words differently from Rashi.

What they mean to say is that the dolphin has relations in a similar way to people; they mate belly to belly, unlike some mammals where the male has to mount the female from behind. Moreover, the female has some form of breasts and it nurses the young, like people. This would fit with the modern dolphin as we know it and this is how the Musaf Aruch (entry "Dalphan"), Rav Yakov Emden, and Yalkut Shimoni understood the matter.

Yoel Domb

We discussed Dolfinim at length in our Insights to Bechoros 8a, https://www.dafyomi.co.il/bechoros/insites/be-dt-008.htm . (You can find a comprehensive discussion in R. Natan Slifkin's book, Mysterious Creatures, chapter 2.)

Kollel Iyun Hadaf