Thread:Juraj103/@comment-27075564-20171115015842/@comment-24448826-20180212150619

Mechemik wrote: ...if Shee'naz is a siren, she looks very different from the ones in game. Aaand I managed to find the reason. This is because The Witcher 3 sirens aren't the real sirens but rather creatures called "nixa" -- according to Polish TW3 artbooks. For further info regarding this see |here discussion.

Conclusion is:
 * Siren/Havfrue/Mermaid = sentient humanoid race (e.g. Shee'naz)
 * Nixa/"Siren" + Ekhidnae = their hostile beastly cousins (e.g. Melusine)