They are sirens, not sea ponies, since twilight said so in the beginning of the movie. This was made before Rainbow Rocks premiered, when all we knew were ponies on the front of toy boxes represented the dazzlings.
Sirens were sea creatures too, and their demonic form looked half-pony as well. So I guess Dazzlings were Sea Ponies who somehow invented/found that negative-energy-sucking necklaces Hopefully it will be explained further in next season/movie.
Let's try and make it right, Don't wanna start a fight And we're so sorry if we give you all a little fright We're not so scary if you see us in the daylight You'll be so happy Just as long as you survive the night (The night... If you survive the ni~i~i~ight)
Oh, I'll take you away To our enchanted land of play..
Forgive me for being suspicious, mischief's not on my brain. We're programmed to be pragmatic if someone messes with the mainframe. It's not that we don't trust you, we do! We love you, too! <3 It's just that here at Freddy's, heh, we have a few rules...
Let's try and make it right, Don't wanna start a fight, And we're so sorry if we give you all a little fright We're not so scary if you see us in the daylight, You'll be so happy just as long as you survive the night... (x3)
I'm sure you'll survive, Just.. Don't break the rules, and play nice..