Initiate flight reaction, then fight reaction, then cognition.

I would tell her I love the night.  

Do you need help getting out of that wall?

Wanna order some pizza and play co-op Zombies all night?

@sammy0205
 
I didn’t even see that before I made my comment! Strange minds think alike.

Tell her to get the Hell off my roof and come in the front door. (There’s a window with an overhanging section of roof to my right.)

Ah!  
Oh, my apologies your highness; you startled me. (Bows)  
May I ask what warrants this unexpected, but certainly not unwanted, visit?
 
(After that…well, let’s just wait and see how things progress from there. ;P)

Jump through the ceiling and scream. If a large magical fictional horse suddenly appeared next to me, that’s the only possible reaction.

Blink twice and reach to touch her face.

There’s only one thing to do in this situation.
 
“Are you a gamer?”

@Background Pony #D4EE  
Actually that’s really small for a horse.

Confused that a giant horse is staring at me.

I would ask her to play Battlefield 4 with me.

Uh…I’m in my underwear Luna…
 
….If unless….
 
 
.___.

“What the f**k? OUT!”

Greet her with a small nod and back to playing Dawn Of War as the Imperial Guard while singing “Panzer Battallion”.

Say Hello and ask if she could make a formal Equestrian scholarship to pay for college completely.

The only thing I’d want to do is this… i’d do her

@Princess Luna  
no offense Princess, but that face from your avatar is the least enticing thing that I’ve ever seen..
 
nobonerforutoday. :/

@Background Pony #44F5  
You seem to be a nice guy. Friendzoned much?

be confused, because no pony should be anywhere near my height

I would kiss her hoof and say “My dear Princess of the night, your mane flow pretty like the stars in the sky. The gleam of your eyes are more beautiful the moon you contol. And most of all, your wings are elegant than I can ever imagine.” <3

Wonder yet again why people don’t just use the friggin’ forums instead of poopin’ up the front page.

Say hi.

Realistically freak out a formally fictionally pony just showed up.
 
What I wish I would do: say “Hi Dream Pony!”