This is how Dressing up for Nightmare Night goes in My Little Pony Transformers Prime: Luna Eclipsed.
[The episode begins with Spike, in a dragon outfit, pacing around]
Spike: Ugh, come on, Twilight and Optimus. We're gonna be late for the Nightmare Night festival.
Twilight Sparkle: We're almost done, Spike.
Optimus Prime: Just give us a vorn.
[Twilight puts on her Star Swirl the Bearded costume while Optimus puts on an Alpha Trion costume]
Spike: So, what are you two dressed as?
Twilight Sparkle: I'm Star Swirl the Bearded.
Optimus Prime: And I am my mentor, Alpha Trion.