It's remix of "Yay!" by Fluttershy. "Yay!" is doping for Rainbow Dash in flying competitions in Cloudsdale. It's from My Little...
Original Shingeki no Kyojiin OP: http://youtu.be/XMXgHfHxKVMWhat made me do this, you may ask? http://youtu.be/PDHCLjuOzXYEverything with this song just beco...