Credit goes to Carnival-Tricks

After having his fun with Celestia, he decided to fall asleep on top of his new pony plushie while snickering that he'll use Celestia in his sleep. After falling asleep, he was greeted by Nightmare Moon, one of Equestria's most frightening foes. Yet to Nightmare's surprise, Rivett wasn't one bit frightened, in fact, he already got his special needle ready and turned Nightmare Moon into a plush toy. Much like Celestia, he did the same thing and rid on her. Sometime during the night, Luna woke up from a nightmare, only to find she can't move.
She looked up to discover that she became a diaper and was in utter shock, but also confused that her and Nightmare were in the same inanimate condition. Regardless, Rivett gonna ride that pony while nature calls to use his new Luna diaper with a sinister grin.

End

I said Luna would be next and she's my favorite pony. As for why Nightmare and Luna can exist at the same time, don't know. Magic physics, anyway next up he's gonna humiliate Cadence and Shining Armor.

Rivett belongs to Me
NIghtmare Moon and Princess Luna belong to MLP