“Eeeu.-.u...-..u…-...Whoa!”, remarks Big Mac.

It’s Big Mac’s Mech. This is what chocolate does to my brain.. This image I could keep going with to do a proper version… And I think I will. I like this mech. But that’ll mean tearing out the half-Equestria hill crap and the foreground everything.. Then redrawing all of it…Ah, it can wait…. Here’s the lo-res speedy version for now.

It's called the 'Hellshock'

Other Mechs
Luna’s Mech -> Night Wraith [link]
Lyra's Mech -> Lyrian Brawler [link]
Rainbow Dash’s Mech -> Thunderbow [link]
Derpy’s Mech -> Crossfire [link]
Big Mac’s Mech -> Hellshock [link]
Applejack’s Mech -> Chaos Maul [link]
Fluttershy's Mech -> Razorwing [link]
Pinkie Pie's Mech -> Overkill [link]
Twilight’s Mech -> Adjudicator [link]

Explanation of the image.
Celestia happens to be drinking a ‘Moon Shooter’, which is her special combination of headache tablets, stomach antacid, ginger, lemon and a viciously strong calmative. Often times, Celestia may call it the Moon Shoota, or in haste and anguish, the-Moon-Shoot-her!, which is generally associated with how perturbed she is from Luna’s behavior.

Applejack is preparing a celebratory bottle to be broken over the mecha’s foot in a traditional launching ceremony for this particular, ‘Princesses creation’. The glass container contains a deep red liquid the precise colour of blood. Twilight knowingly spots the item and half hisses and yells to Applejack, swinging a hoof wildly back past her neck in the ‘stop it’ signal, “Icks nay on the..- ‘Tomato Sauce’ ey!”

Luna hearing the odd speech guesses at it’s meaning, “Shall we not break the traditional vial of blu-“. Luna observes Twilight pretend severing her own head with a catatonically distressed look. She continues, “… I mean bottle of demo-.n.-..,oh, ‘Tomato’ - sauce over it’s foot?”

Celestia unobservant and petulant speaks to Twilight whilst gulping her, ‘Moon Shooter’, “You said, Luna designed t-h-i-s model? It looks like a summoned demon. I’d expect it of Nightmare Moon.”

“Ah, well, funny you should say that.”, says Twilight nervously tugging at an imaginary collar.

Celestia twists about with her patented look of disapproval.

“hee.. hee… er, just joking..”, says a withdrawing Twilight.

Twilight begins the countdown until Celestia’s calmative kicks in…’10…9…8…’

***(Just a note: I'll be fronting up to the Armageddon Expo in Melbourne this year, down in the Artist's Alley, so if you want to see loads of mecha, space stuff and all sorts of groovy things, come along for a visit. I'm probably going to bring one of the Wacoms too, so I can teach others to draw them)