The Dragon Lord and King of the Changelings have been invited to the Royal Canterlot Gala. They're allowed to bring one extra creature, so they choose Garble and Pharynx. Those two should be able to get along, right?
Pharynx won't ever accept these new Changeling customs. He won't ever accept the lovey-dovey ways of his peoples. Even on his death bed he won't ever sympathize for the Ponies like his people do. Maybe he'll tolerate, for the sake of Her.
Pharynx doesn't really like potted plants, but Princess Luna does. So what better way of getting him to stop ruining her lavender pots, if not with a good lesson or two?