Sirens

Sirens are creatures from Greek mythology. They are a mixture of a woman and a bird in appearance, though exactly how the mixture looks differs greatly between accounts. Because sirens are associated with the sea, they are sometimes als depicted as mermaids. The song of a siren is capable of enchanting either humans or men specifically, which is usually used to lure them to their death.

Monster High
The Monster High phantoms are Melody Carver and Ms. Siren. There's also a Create-A-Monster iteration of a siren. Between the three, Melody is birdlike and the Create-A-Monster fishlike. Ms. Siren's appearance is unknown. It is thus not clear what sirens in Monster High are supposed to look like, though characters resembling the fishlike siren, such as Sirena Von Boo and Rider, have been since been identified as mermaids.