Based on this scene: [link] [link]

Also, if you don't get who the third pony is ... [link] ;)

My little Pony: Friendship is Magic is (c) :iconfyre-flye: and Hasbro

Not sure if the title "Making sense of Pinkie Pie" is the most fitting, so if you got a better idea (that doesn't spoil the ending of course), fire ahead!
EDIT: Just for the record: Changed the title. Think this one's more specific.