Pinkie's played quite a few instruments over the course of the show. In particular it was Pinkie playing trombone that motivated me to pick up my trumpet and record something. From there came this track.

The first 20 seconds are a sort of sonic 'montage' of Pinkie's various attempts to play instruments. There's the trombone and the sousaphone from the parasprite episode, and of course the yak instrument thing ('yovidaphone') from that yak episode. So this song is something of a celebration of the various musical efforts of Pinkie.

On a slightly more serious note, Pinkie has changed and developed across the many seasons of this show. I haven't watched season 9 yet (no spoilers pls) but I certainly hope that it brings the journey of the pink horse to a satisfying close.