Even if today's mermaids are described as benevolent, we must not forget that in ancient times they used their wonderful voices to hypnotize sailors so as they could sink and they could eat them (in French mermaids and sirens are the same word, "sirène" so to us they belong to the same species, we just see that in older legends they were bad and newer they are good, or that some are good and others bad, I learnt the difference in English only this year). In Greek mythology they were half-women half-birds but progressively people made them half-women half-fish and their myth spred in all Europe. I like to think like many they have tails of different species of fish, I drew one with a shark tail to enhance her ferocity.

Sirens © European folklore (originally Greek mythology but with bird bodies)
Drawing © me