Melmedas, by Farima Keshavarz, Eranshahr website. Iranian.
I found a possible branch of the two-tailed siren's family tree, Melmedas:
"Melmedas is a mermaid-like creature. She has a very beautiful and attractive upper body. But in the lower part of her body she has two large, sharp and very dangerous sickles. In some areas of the (Hormozgan) province, it is believed that her hands looked like sickles (Hinnells 2009). Melmedas would appear to the youth and invite them to sleep with her, but while sleeping, her legs would cut the young victim into pieces." (Avarand 2014)
The website Eranshahr has some more information:
“The people of Hormuz Island believe that she resides next to the salt valleys, by a big konar tree (Persian ironwood tree). Under this tree, a beautiful woman sits around sunset, whose name is Melmadas. She combs her long hair and the men who see her fall in love with her charming look. When men start talking to her, Melmedas approaches them and suddenly changes face: Old-looking woman with legs like sickles. With sickle-shaped hands, she cuts the deceived men to pieces.”
This sounds like a close cousin to sirens and mermaids, who like to lure in men to kill them.
I also found her imagined by another female artist:
Bianca Tschaikner, Austrian.
From her website, which has many lovely Melusine, Melmedas, and two-tailed siren art for sale:
“But now I know that two-tailed mermaids live on this (Hormuz) island as well. What fascinates me about the Melmedas is that unlike the Melusina, two-tailed mermaid from Northern Europe, who has spread her tails joyfully in a gesture that feels almost sexual, her tails are folded over each other, which gives her an air of reserve, of prudence, maybe of hostility. They remind me of scissors. Like other female water spirits, embodying lust and danger, the Melmedas favorite pastime is to entice men. They appear in the shape of a beautiful young woman, hiding her two fishtails under the water. Once the man is in her net, turns into an ugly old woman and rips him into pieces with her fishtails. I wonder what the fishermen from Hormuz would have told me about them.”
I wasn't able to track down the book Avarand mentions, unfortunately. So I'll need to keep digging. Worth noting is the oldest two-tailed siren is from Luristan, in now modern Iran.
Avarand, Somayeh. “An Anthropological Study of Melmedas in Iran and Siren in Greece.” The Anthropologist, Kamla Raj Enterprises, 2014.