Rainbow is fascinated with Fluttershy's tail for some reason. What is it?

Comic by DeusExEquus
https://www.deviantart.com/deusexequus/art/Tail-Trick-633158902

Agent0Fluffy as Fluttershy, Applejack and Rainbow Dash

Music:
The Underpants Theme: South Park Stick of Truth
That Thing We Do 1221/2
Mike Stobbie (PRS)
Music from Epidemic Sound (http://www.epidemicsound.com)