so i'll give a small explaination of the diffrence
A Siren is a Mermaid looking Creature that disguises itself as a mermaid
In ancient times, and up through most of history, a bird-looking creature with a woman's head. Closer to common era calender, a woman with the wings and legs of a bird. The most famous mention of them, in The Odyssey, neglects to describe them(though all the contemporary art is bird), so these women that appear at sea, though perched and not in the water, are often associated with the sea and gets mixed in with myths of other sea-women. But that still wasn't a common idea for a very long time. Even after a thousand years, the bird qualities were common in depictions, but started to become a mix of both bird and fish. Wings, clawed feet, but a fish-like tail too.
Today, too, there are still modern depictions that choose the mix of bird and fish, but mermaid-like comes up a lot. I personally like the hybrid design, a woman of avian and aquatic. It's more interesting. We can keep the purely bird monster girls to the harpies and the fish can stay the mermaids. Sirens get a better niche by being a combination.
