With the debacle from https://derpibooru.org/2402913 out of the way Pinkie Pie was surprisingly eager to undergo the miniaturisation process herself. Down to just two ponies at this point it was Rainbow Dash who got herself a cheerful little pink slave, although her main goal was more to get the sense of being magnificent in the eyes of another.
 
Pinkie Pie was ever so keen to serve, which Twilight identified as being her own quirky nature amplified by the subservience aspect of the spell. Rainbow Dash did appreciate her tiny friend’s enthusiastic scrubbing of those hard-to-reach spots out of the way of prying eyes that she figured would be more likely down the spa.