A short crossover I wrote up on Tumblr.

I really love Monsters Vs. Aliens and want to cross it over with Transformers, but I just haven't thought of anything until now.

Disclaimer: I don't own Monsters Vs. Aliens or Transformers, and don't make claims to either series or profits thereof.

Ginormica leaned down, sufficiently large that her foot was slightly bigger than the car she was using as a foot rest, and poked the metal dinosaur with her finger.

For something that had survived atmospheric reentry (which, Dr. Cockroach had assured, made no dang sense at all), it was in remarkably good shape. where she touched it, it felt cool to the touch. She looked down at it, the robot tyrannosaur composed of variously colored machines and metal assembled into its saurian form, complex and intricate and totally self-contained. Her hand lingered over a peculiar shape on its hip, that looked something like a stylized noble and mechanical face.

Her fingers, each individually larger than a person, trailed against the metal skin. It felt warm, not with heat from atmospheric re-entry that had turned its crash site into a molten mess of vitrified shards and fused earth, but with something from inside. A regular pulse, as if of an electric heart.

The trembles moved into her hand, and the giant woman bit her lips, white hair moved over her ears with a sudden wind. The mechanical tyrannosaur felt… alive.

"Guys?" Ginormica said into the special communication earpiece built for her scale, slightly larger than a person. "We got an alien… an alien robot. It looks like a dinosaur." Squabbling in her hearing. "A Tyrannosaurus Rex, I think. No, I don't know how an alien machine looks like a dinosaur, maybe it's just really old… hang on, I'll take a closer look at it."

The long and lethal head twitched, and Ginormica jerked back, landing on her backside with an impact that made trees snap right off their trunks. The robot dinosaur's eye fluttered, and then it snapped open. He growled, a deep bass that almost sounded like words, and he forced himself up with the puny arms tyrannosaurs were so well-known for.

Slowly, powerful mechanisms that were either hydraulics or synthetic muscles or an efficient combination of both powered up in its hindquarters, pressing it up. The tyrannosaur robot stood up to its full height, looming over Ginormica, and as its glowing blue eyes took fierce notice of her, its tail straight behind it, Ginormica squealed into the communication array extending invisibly over her mouth, "Scratch that, it's alive!"

The robot dinosaur narrowed its optics at her. It's jaws opened, revealing rows upon rows of sharp teeth, and a light within its throat. Ginormica rolled to her feet, ready to shut its mouth shut if it fired a beam, and to her surprise, it said (in an extremely deep voice with a bit of a thick rolling over the more articulated words), "Me, Grimlock, not an it!"

"Guys, it… he… he talks! He's sapient!" Ginormica said. B.O.B. asked something on the line. "What? It means he's a person!"

The dinosaur - Grimlock, it seemed - snarled at that. He stomped forward and then to Ginormica's amazement he… well, transformed. That was the best word for it, the mechanical behemoth's entire body reconfiguring itself, joints and plates dissembling as the outer frame repositioned itself, sliding back into place in a much different form, traces of the dinosaur shape evident here and there as it adopted a much more humanoid configuration.

The resulting giant robot wasn't quite as tall as Ginormica; she was pretty sure that he wasn't much more than half her size. He was a lot wider, though, form bristling with spikey armored plates and weapons both part off his very being and wielded by him. His tail had transformed and split from his body, morphing into a massive sword proportionate to his form and brimming with energy that was not quite lightning or liquid.

A grim face glared up at her, a tinted visor instead of readily visible optics, and if there was a mouth, it was concealed under a blank faceplate that seemed part of his body.

He stared at her, presumably thoughtfully. "Me, Grimlock, remember humans being much smaller." He considered. "Also, puny."

Something moved, under that faceplate. She guessed it was a savage smile.

"Me, Grimlock, fight you," He said bluntly. "Me, Grimlock, bored!"

"Wait, what?"

Grimlock dropped the sword to the ground, as well as the variety of weaponry he carried, the rest retracting into his body. He rushed at her, screaming like a monster out of primeval legend, and when Ginormica slammed her shoulder into her (winced that it actually hurt), he roared with satisfaction.

When Doctor Cockroach, Missing Link and B.O.B finally arrived on Insectasaurus, it was to find Ginormica and Grimlock sitting together in a totally flattened forest, Ginormica nastily bruised and limping on one side. Grimlock's armor was battered and a few plates were missing, and part of his optic visor had been cracked. He seemed happy, though.

"Hey, guys," Ginormica said, grinning and missing a tooth or two. "I think I made a friend."

Grimlock gave her a friendly punch. "Now you me, Grimlock, battle-sister!" He roared to the heavens. "Puny humans not so puny! Me, Grimlock, approve!"

"…Huh," Missing Link said. There wasn't much else that could be said.