Page 66 — The Worst Possible Thing
Out of all the things Nightmare Moon could've done to impede the heroes' progress, why attack a mustache?
I imagine the scene went thusly: Nightmare Moon came across the river as she raced ahead and thought it'd make for an insurmountable obstacle if properly prepared. All she'd need to do is rile up the local sea serpent (it's a wild river in the Everfree Forest; a sea serpent being present is a reasonable assumption to make). It wouldn't be difficult — she'd already done something similar with a manticore, and an aggressive sea serpent would be even more dangerous.
In short order, she finds the sea serpent of the river and whisks past him sharply, only knocking off a few hairs to get his attention. It definitely takes notice. She gets ready to attack again to further agitate the creature… when it breaks down into tears, lamenting the loss of its mustache. He throws such a fit, such great heaving sobs and tantrums, that the river is made almost impossible to cross by his thrashing.
Not what she expected at all, but… mission accomplished, one way or another. With time being of the essence, she moves on and leaves the serpent be.
And that's how Steven Magnet got his mustache chopped off.
Transcript:Twilight Sparkle: Alright. I say: Excuse me, sir… Wait, is it “sir”?
DM: …Would you like to roll an Insight check?
Twilight Sparkle: Nnngh… Excuse me, SIR. Why are you crying?
Steven Magnet: Well, I don’t know! I was just sitting here, minding my own business… when this tacky little cloud of purple smoke just whisked past me and tore half of my beloved mustache CLEAN OFF! And now, I look HORRID!!
SFX: (BOOSH!) (SPLASH)
Applejack: So… A “madam,” then?