That would be the fourteenth episode, the one with Vinyl Scratch in it. I like the musical parts, very catchy. Same with the musical parts in every episode except for two or so songs from Pinkie Pie, but those songs were meant to sound rather annoying, since the ponies in the episodes radiated their distaste towards those songs. The songs that I'm talking about are the one in episode twenty-one and another one that I forgot.
So, what's up? Haven't been in the main thread much, perhaps just because of lack of things on-topic to say. I am absolutely overjoyed that Scootaloo's apparently getting an episode at last.
Actually, it was a talented troll in ponychan who came up with that while posing as one of the animators for the show (among many other stuff, like S3 being longer than the confirmed 13 episodes) so no clue if there's a Scootaloo episode yet.