Sirens (My Little Pony)

Sirens are creatures from My Little Pony Equestria Girls: Rainbow Rocks.

Sirens, namely Adagio Dazzle, Aria Blaze, and Sonata Dusk, originally roamed Equestria causing disharmony with their song, feeding off negative emotion. The more negativity, the more powerful they become, with intention to conquer Equestria.

They were banished to the human world by Star Swirl the Bearded, believeing they'd do no harm there, unaware that Twilight Sparkle and Sunset Shimmer would bring magic to the human world.

Their true forms resemble that of a hippocampus, another mythical sea creature.

Fire Rebellion Storyline
The Dazzlings caused some trouble in the human world of Canterlot, which tugged the nerves of varrious Order of the Just entities, namely Agent Michigan and the Brotherhood of Steel. The Brotherhood, feeling that there's more than just those three Sirens, view the Sirens as a much greater threat than the Synth.