Classic sirens have not always been the beautiful half women and half fish beings. The first descriptions characterized them half woman half bird, and they were daughters of Achelous, the Greek spirit-god of the homonymous river, or god Phorcys, and of the nymph Calliope according to some versions or of some muses according to others. These malicious birds were the same ones that tried to seduce Ulysses and his crew with their intoxicating song in Homer’s Odyssey. However, in Greek mythology there were also women who inhabited the sea, the Nereids, and a half-man half-fish god: Triton, son of Poseidon.
It is not clear when the metamorphosis happened, but something may have to do with the manuscript Liber Monstrorum (7th or 8th century), elaborated by the Anglo-Saxon Audelinus, who identifies mermaids as beings half woman half fish.