On Tuesday the Daf Yomi learned Bechoros 8. At the top of the daf the Gemara says הדולפין פרין ורבין מבני אדם (Rashi's גירסא is מבני). Rashi explains this as follows. דולפין is a sea creature that looks half like a person and half like a fish, they are called sereine in Old French. Rashi clearly refers to mermaids (the French sereine derives from the Latin siren, meaning mermaid). The Gemara is saying that if a man and a mermaid have relations the mermaid can get pregnant.
The truth is that the simple pshat in the Gemara is that it is referring to dolphins and the Gemara is saying that dolphins reproduce like people ( הדולפין פרין ורבין כבני אדם).
However, Rashi for whatever reason had a different girsa (see Tosafos there) and believed that the Gemara was talking about mermaids.
What we see clearly from Rashi is that he took the mythology of his time and believed it and used it to understand the Gemara.