...initially, at least. The final design is probably Power Rangers-inspired.
http://dl.dropbox.com/u/4152380/MLP_UltraHORD.jpg
UltraHORD, merger of six HORDs (Harmony Operated Robotic Destroyers), plus addon.
Twilight - Magenta Ranger, piloting the Harmonic Unity HORD, forming head and torso structure.
Applejack - Orange Ranger, piloting the Harmonic Justice HORD, forming the right arm with dual cannons and hydraulic ram.
Rarity - White Ranger, piloting the Harmonic Benevolence HORD, forming the left arm with the power drill and energy whip.
Fluttershy - Yellow Ranger, piloting the Harmonic Mercy HORD, forming the left leg with jump jets and EMP emitter.
Rainbow Dash - Cyan Ranger, piloting the Harmonic Tenacity HORD, forming the right leg with jump jets and Sonic RainStomp/Roundhouse SonicRainBuck attack systems.
Pinkie Pie - Pink Ranger, piloting the custom "Party Animal" Harmonic Enlightment HORD, forming the backpack with dual shouldered Party Cannons and assorted support items.
Celestia and Luna - present as the unpiloted Harmonic Balance HORD, forming the breastplate, shoulderpads, and assorted armor pieces on the machine.