So this happened because of the pondering you'll see below.
I don't know why, but it just kind of... came to me.
"I think so, Brain, but it's a miracle that this one grew back."
For a moment, Brain went incredibly quiet and his mind traveled back.
Brain had been attempting to take over the world since somewhere around his ninth month on said planet. His first birthday had come and gone with little fanfare but for the cake (a food pellet with a drawing of what was supposed to be a match draped across the top) that Pinky had provided. And, rather than the typical Happy Birthday tune, he'd been serenaded with Twinkle Twinkle Little Star.
But as the second drew near, their nightly plans grew more and more dangerous. Not for The Brain, of course, but for his associate. The inane, ridiculous creature clearly had no concept of danger and a staggeringly high tolerance of pain. So much so that it bordered on masochism, the taller mouse giggling at every bop and every harsh strike delivered to the top of that empty head.
Therefore, it was without qualm that the megalomaniac brought forth their latest plan: they were going to replace a single Vienna sausage in every can with one that would cause strong addictions to the miniature hot dogs. And then, as the worldwide addiction grew, the mice would be able to insert another additive that would make them susceptible to suggestion. With ease, Brain would then be able to take over the world!
But the plan had to be implemented soon or the sale of the miniature food would no longer be as popular as it was during a particular sports season. Why humans had parties for the viewing of a single game was beyond him, but he would use the oddity to his advantage.
It was in the cannery that the danger became apparent and the taller mouse finally spoke up as every can of sausages was sealed on a conveyer belt in front of them. "Wuh- But Brain, isn't going in and out of the can before they get all closed dangerous?"
"For some mice it may be, yes. But I'm sure that you'll be fine. Your speed, agility, and simple dumb luck will be enough for you to go in and replace a single sausage. You'll surely have enough time to flee again before the can is able to be sealed."
"You think so, Brain?"
"Of course, Pinky." The smaller mouse gave an impatient wave. "Besides, I'll be at the control panel. I'll press a button at exact intervals, allowing you an extra three seconds per can lest you become confused. Understand?"
He saluted, hefting a very large bag of hypnotic foods. "Right, Brain! I'll do real good!"
"Yes, I'm sure." Brain gave a dismissive wave of his hand and left Pinky behind to complete his task.
And the plan went smoothly for quite a while. Grab a sausage, switch it with another, drop the sausage. In and out of can after can, with Brain pushing the button exactly every single time. Until his excitement grew. He tried to fight it back, clinging to his dignity and marking time with an exactness he took pride in. But the anticipation of success still bubbled in his chest, swelling it. And he tried very hard not to succumb to it… which, in itself, was the plan's downfall.
He was trying so hard not to let his hands tremble with the excitement that he focused on them and forgot the button. But the scream stilled his hands and everything else in his body as he looked up. He'd never before heard that sound out of that mouse, so was down from the control panel and sprinting across the floor to get to the conveyer belt his friend had been stationed at. He stopped abruptly when something damp dripped onto his head, and was simply horrified when he lifted a hand to wipe it off and found the red on his fingers. Pinky!
"Braaaaaain!" he wailed and the megalomaniac ran up, gasping at the sight before him. Pinky was being carried along the conveyor, unable to run away because one of his legs was… Breath hitching, Brain grabbed his friend and dragged him away from the growing pool of blood. There was so much of it, more squirting from Pinky's leg with each heartbeat. He'd clearly severed an artery. Brain ran for the bag, grabbing a tissue, and scrambled into one of the machine to pry something away that could be used a clamp. All the while, Pinky's whimpered squeals grew softer and softer. He was losing too much blood.
The smaller mouse scrambled back to him, applying pressure and hating the sharp squeal it induced. "Shh, Pinky, just hold on. It'll be okay." It was a clean slice from what he could see, just above the knee. And it made his stomach roll. It was worse that he had to reach in, latch onto the spewing artery and tie it off. Pinky screamed, back arching. His eyes rolled back into his head and Brain bit his lip hard to keep himself from stopping. He needed to do this, he needed to heal his friend as best as he could hear until they could get to the lab.
Fat tears were rolling down his cheeks now, and Brain now cursed Pinky's high tolerance for pain. Any one else would have passed out long before this, but not Pinky. Oh, not his friend. His only friend. "Pinky, you'll be okay." He leaned over, pressed their noses together, and got a whimpering babble in response. "I know. I know it hurts."
"Wh… what hap-p-ppened, B-Brain? I th-thought I was d-d-doing it right," he managed, those big blue eyes staring into Brain's. His smarty bestest best friend would know.
Guilt tore at him, clawing at his heart and squeezing it angrily. Pinky had been doing everything right. It was The Brain who had made the mistake, who had let his attentions falter. And with teary eyes slowly glazing over, Pinky finally succumbing to blood loss, Brain let a tear of his own plip onto his fur. "It was an accident, Pinky…"
"'Kay." It was all the explanation the taller mouse needed, his trust of Brain so absolute that he simply didn't require the details. "It's raining, Brain…"
"I know, Pinky." He couldn't stop the tears, managed to get the artery tied and clamped. "Just go to sleep. I'll get you to the lab. It'll be alright." He put as much pressure to the cut as he possibly could with a tissue, ran to the bag and emptied it without preamble and dragged it back over. He lifted Pinky gently, so gently, and settled him down on top of it.
"Brain… I feel heavy… Everything's heavy…"
"It's okay," he repeated, brushing a hand over Pinky's brow, using the strap of the bag to keep pressure on his leg as he quickly dragged him away from the blasted cannery. What had he done? What had he done to his dear friend?
He continued to ask himself that question through the night, fighting desperately to stop the bleeding. He had the artery closed, a medical book opened to a page on… on amputation. And shivered as he continued. If he'd grabbed the can, if he'd thought to, then this wouldn't be like this. He could've figured out how to reattach the limb, but now someone else was going to end up with a mouse leg in the can.
It was almost funny, imagining someone's reaction when they opened the can expecting miniature hot dogs. The cannery would probably be sued, shut down. It was… It was almost funny.
The Brain worked tirelessly through the night until there was nothing left to do but sleep. Pinky's leg, or what remained of it, was bandaged thoroughly and Brain was largely covered in blood. But he staggered back and dropped, curling into a ball and weeping on the floor of their cage. He'd taken Pinky's leg for naught. The closest and sole friend he had, losing such an important piece of himself to assist in global conquest. The shame of it stole his breath and kept him from sleeping long past the point of exhaustion.
The shame of it drove world conquest from his mind for days while he and Pinky both tried to adjust to his lack of mobility. He looked at the wheel longingly, even tried hopping on his one good leg to get to it once, and cried when Brain made him get down. He cried even harder when the smaller mouse unscrewed it entirely from the side of their cage.
"I don't hurt anywhere, Brain! Please let me run! Don't take my wheel, Brain! Puh-leeeze!" Relegated to the bed, the taller mouse was unable to do anything but beg and plead and make Brain feel more horrible than he already did.
"It's for your own good, Pinky," felt bitter on his tongue.
"I'll never get anywhere without my wheel, Brain!" he sobbed, sobbing harder when he reached down to scratch the leg that wasn't there.
Brain took far longer than necessary to hide the wheel in the basement.
The cage was quiet as more days passed, the scientists not even taking them from the cage once they'd seen Pinky's missing leg. None of them knew what had happened, each expecting the other of removing it and not admitting to it. But Brain knew the truth and it kept him up at night. He no longer shared the bed with his cagemate, hiding in a corner of their cage to hide from the whimpers of, "Brain, my leg hurts" or "It itches, Brain" or, worst of all, "Will it come back tomorrow?"
It would never come back. Limbs didn't just regrow. At least not with mice. So Brain hid and tried not to hear the soft crying that made every piece of his body ache with guilt and shame. Pinky was so happy, always so happy. But sadness hung in their cage and pressed upon the smaller mouse like a physical weight. He wanted his happy, inane friend back… But how?
The prosthetic was a failure. Oh, not because Brain hadn't designed it specifically for his friend and made it as realistic as possible. But because Pinky was a dancer. Even when he walked it was with a special kind of grace. The way he was forced to move with the metal attached to his leg didn't work with his natural flow. And it was heavy. And it made his leg itch worse, Brain. Why can't I have my leg back, Brain?
The megalomaniac fled, hiding behind a stack of books with his body curled into a tight ball and his hands over his ears. The words echoed in his mind, the unanswerability stabbed at his heart. He was a monster. Never before had he hurt anyone, but he'd robbed the happiest mouse in the world of happiness. He was a monster.
The cane worked surprisingly better than the prosthetic because he was able to convince Pinky that it was a game. Pinky had taken it to mean that if he jumped around enough with his fancy cane that his leg would get jealous and pop out from its hiding place. Brain didn't correct him, couldn't. Because he'd seen that smile and the hope that shone so brightly in big blue eyes. He couldn't take it away, so let them both live with the lie.
And for a few days, the sadness wasn't as unbearable. Though he still stayed far away, unable to sleep in the same bed with his dear friend. He missed the warmth, but he just couldn't do it. Not when Pinky still reached to scratch what wasn't there.
But then Pinky hobbled over and the blue eyes that had been hopeful for days were damp. "Brain…? How come we're not friends anymore?"
He couldn't answer, tongue tangling and tying over every placating response he could come up with. And, with his IQ, there were a lot of things that occurred to him in just a few seconds. Finally he swallowed and tried to tell the truth.
Except for one thing that stopped him dead. "Is it 'cause I can't help you take over the world anymore? 'Cause I messed up the plan and got broken? I tried to tell it to come back, Brain, and it won't."
The tears began to fall then, but from pink eyes rather than blue. "P-Pinky…"
"I'm sorry, Brain. I didn't mean to! It doesn't itch anymore! I promise!"
Oh, it itched. Brain could see it in the way Pinky's fingers kept twitching towards the air where fur and flesh should've been. But the words beat at his already battered system, his emotional roller coaster dipping far too low. He stood and reached for his dearest of companions and held him close in a very rare hug. "It wasn't your fault."
"Then how come we're not friends, Brain? I want to be your bestest best friend and have tomorrow nights!" Blue eyes still shimmered with unshed tears, but Pinky wouldn't dream of crying when Brain needed comfort. He used the hand not bracing on the cane to cling to the smaller mouse, but a frustrated sound that had become commonplace on the formerly upbeat mouse slipped out.
He couldn't hug like Brain needed to be hugged. He couldn't sweep him up and hold and squeeze. He couldn't come and get Brain at night when he fell asleep in the wrong place. He couldn't sweep his friend up in a dance when he needed to smile, not that Brain ever really and truly smiled, but this was the worst not smiling ever. He couldn't dance at all! And he couldn't even run on his wheel because it was all gone.
Pinky bit back his sniffles and lowered his head to rub his nose against Brain's brow, not even smiling when his ears fell back. "D-don't cry, Brain. I'm sorry. I'm really sorry!"
Brain escaped the embrace with an ease he never would've managed before, wiping hastily at his tears. Pinky was just so... so stupid! "It isn't your fault, Pinky! It was never your fault!" And the admission he'd been holding so tightly burst. "It was my fault! I didn't hit the button! I didn't give you enough time! I don't deserve you!"
It was sometime after that, as he'd fled immediately and not given the lanky mouse a chance to respond, that he'd claimed not to deserve Pinky, yet he'd said nothing about the world. It just reminded him of their first Halloween together, when Pinky had been taken to Hades after selling his soul to give Brain the world. He'd refused to have the world without Pinky then, and it seemed to hold true still. If he didn't deserve Pinky, how could he deserve the world?
The question kept him up for the next several nights.
When Brain realized that Pinky wasn't sleeping in their bed any longer, something in him shattered. He marched over to his friend, found him curled around his cane, not asleep and shivering on the floor below where his wheel had once stood. "Pinky, why aren't you in bed?"
"If you don't like the bed anymore, neither do I," was the tired, stubborn response.
His cagemate needed to be in bed. It was safer and far more sanitary, so Brain coaxed him to his, his breath hitched a bit, to his foot and helped him get to bed. He had no choice but to stay with him that night and, for the first time since the accident, slept through the night. The warm presence was so blessedly familiar, chasing the heaviness away from the cage and away from his mind.
He awoke with an idea that was too stunning to try.
But he had to try.
There was a mouse in the other side of the lab who had lost his hand to an experiment ages ago. Brain had been on the threatened end of the hook his hand had been replaced with more than once, the creature being an angry, vile sort. At least he was in Brain's opinion, but most were if they weren't Pinky.
Brain waited until night fell and his cagemate fast asleep before enacting the treatment he'd developed. It was a mixture of stem cells and various other chemicals, including a piece of DNA from a lizard and the angry mouse. Nothing happened. He went back every night for a week. Nothing ever happened.
He gave up and while Pinky was starting to smile more, Brain suspected that it was only to cheer him up. He'd been trying very hard not to hear Pinky ask why they weren't friends again, had therefore been spending as much time with the taller mouse as possible. They were friends and they always would be. No matter The Brain's guilt, he didn't want to cause more tears or more doubt.
Yet Pinky made him flee again soon enough, only because Brain didn't want Pinky to see tears either, lest that encourage him to shed his own. "Brain? It's okay that you didn't press the button like you weren't supposed to. You're still my bestest best friend and I'll be very happy for you when you take over the world!"
He ran all the way to the other end of the lab, heard the excited commotion from the scientists, and couldn't help but investigate. Perhaps their stupidity would be enough to distract him from-
The hand had grown back. Miracle, they were saying. Miracle, miracle, miracle. Full mobility. Miracle.
He could use a miracle, so ran back to his own cage.
Undoing the bandages was the worst part for both of them. Changing them was always cause for winces and tears. "It is back yet, Brain? Tell it to come back out. This isn't a very fun game at all."
He dripped some of the healing concoction onto the stump of leg that remained, ignoring the queries his cagemate posed. "What's that, Brain? Is it medicine? Will that make everything all better? Will it be back tomorrow? Can we have tomorrow nights again?"
And when the leg grew back, Pinky cheered and danced and twirled and and finally scratched that itch and swept Brain up for a tight hug even though he didn't understand why his bestest best smarty mouse friend was crying so loudly. He'd made it all better! Telling him that led to even louder sobs, so Pinky had danced with him until the tears had ebbed.
He danced more when his wheel was returned, sweeping his friend up in another dance, and smiling brightly at the whispered promise. "This will never happen again, Pinky. My plans will never cause this much damage again."
"Right, Brain!" He giggled, blue eyes bright with joy, and the cloud that had hung over their cage dissipated. They had their miracle.
But it was alright now. His plans weren't dangerous any longer. At least the ones they implemented weren't, and the ones he thought of that posed a threat to his friend were hidden away in a sardine tin. He would never allow such a thing to happen again, just as he'd promised. So the megalomaniac rolled his eyes and focused on that night's plan. "No, Pinky."