Sirens are thought to be the evil or dangerous counterpart to mermaids...
First seen in Greek mythology, sirens were thought of as dangerous creatures. Their beautiful singing would lure sailors to rocky coasts and ultimately to their deaths. Now here is where history of sirens differ to the history of mermaids. Sirens were only ever found in Greek mythology whereas mermaids were folklore from all around the world. Sirens are half woman, half bird and like mermaids were all female and incredibly beautiful but unlike mermaids who were seen as being kind and helpful towards humans. Sirens wanted to lure and drown the sailors.
As time passed, people began to combine the legends and suddenly sirens became the dark version of mermaids. Sirens didn't lure sailors to drown like myths said, instead sirens would eat the sailors. They are what are being depicted as mermaids in Hollywood today.
The actual mythology of sirens make them look more like what we would refer to as harpes.
Regardless, they are still a pretty cool myth and I though I would share some facts that I found out today.