You sure? I've seen every single episode of MLP, and I don't remember it. I remember them in their Dragon/Siren forms in the beginning of Rainbow Rocks, when Twilight was telling the Mane 5 about the Sirens and how they took negative energy from Equestria, but they were only in their dragon form in a book, in a picture. And then at the end of Rainbow Rocks, their Siren forms were shown, but they weren't REALLY in that form. i'm not really sure what it was, but i think they were becoming Half Dragon Half Human, just like how the HuMane 7 become Half Pony Half Human whenever they sing. Only difference is that there was their actual Siren forms, but I think it was a hologram coming out of their pendants, when they had a "Real Battle of the Bands". But I swear I don't recall them ever appearing in anything aside from Rainbow Rocks. I'm pretty sure for now, they're exclusive to Rainbow Rocks, unless of course they make an appearance in Friendship Games or something like that. I really don't remember there being an appearance by the Sirens in the actual TV show. Heck, Sunset Shimmer is never mentioned in the TV show, so why would the Sirens/Dazzlings be any different? If you really think the Sirens/Dazzlings appeared in Friendship is Magic, please tell me the name of the episode. You may be talking about the episode when Spike left Ponyville to live with his kind, and Twilight, Rainbow Dash, and Rarity went after him. If that's what you're talking about, then I'm 100% sure the Sirens aren't in that, though there ARE other Dragons in it.