Aaahhhhhhhh I'm gonna feel so guilty about this one! AngelTale Sans is so cute! ;-;
Anyways, this one was requested by lokiplayz
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Sans let out a sigh as he stared off into nothing. He flapped his huge feathered wings, the wind blowing softly through the soft light blue feathers.
He always enjoyed a nice, peaceful flight through the sky every once in awhile. It was his release, one might say.
Sans slowly began to descend back to the ground below. He should really be getting home. Paps would be worried about him.
As he got closer to the ground, Sans became aware of a faint burning pain in his chest. At first, he simply shrugged it off. It was probably nothing.
Once he had landed, Sans began to head back towards his house. As he trudged through the snow, he was dimly aware of the pain in his chest growing worse.
   By the time he made it back to his house, the pain has become unbearable. Sweat poured from Sans's forehead as he knocked on the door, his arms weak and tired. His wings drooped heavily due to the growing exhaustion.
The door opened shortly after Sans knocked. Papyrus looked down at his brother, a look of concern in his eyes.
"Brother? What's wrong?"
Sans panted slightly as he stumbled through the doorway. After a few feeble steps, Sans fell down, Papyrus catching the smaller skeleton before his skull hit the ground.
"Sans?"
Sans winced as the pain flared up in his chest. "I feel so weird... Everything hurts..."
"What do you mean?" Papyrus asked, his eyes dark with concern.
Sans grunted softly as pain surged through him. "I-I dunno... It just... hurts..."
The smaller skeleton shivered, despite the intense fever-like feeling that seemed to be spreading through him. Sans's wings instinctively wrapped around himself, as if it would shut out the pure agony that wracked his body.
"Well... Lets just go sit down on the couch. Maybe you're just tired?" Papyrus suggested.
"I dunno if I can w-walk that far..." Sans mumbled.
"It's okay. I'll help you," Papyrus insisted, grabbing Sans's trembling hand and helping him up.
He allowed the smaller skeleton to lean on him for support. They had barely made it halfway to the couch before Sans suddenly doubled over with another pained grunt, this one slightly louder than before.
"Are you okay brother?" Papyrus asked.
Sans took a moment to catch his breath before answering. "It's getting worse... I'm scared... Oh god, it hurts..."
He gripped the collar of his jacket, completely unaware that the fabric was beginning to tear beneath his fingertips. Papyrus winced as he felt his brother's grip tighten on his hand. He could have sworn that he'd felt something sharp digging into the palm of his hand.
"Hold on a minute..." Papyrus held up his brother's hand, taking a moment to truly look at it. "Umm, brother? Have your fingertips always looked like this?" He lowered his brother's hand so that the smaller skeleton could see.
"What do you..." Sans trailed off as he really looked at his hand, specifically at the tips of his fingers. They had somehow grown sharper.
"What's going on with your teeth?" Papyrus asked, pointing to Sans's mouth.
"Huh?! What's wrong with my teeth?!" Sans exclaimed, holding a hand up to his mouth and tracing over the gaps between his teeth with a single claw.
Papyrus was right; his teeth had grown sharper. Sans's mind reeled as he tried to piece together what was happening.
Before he could say anything else, a wave of sharp pain shot down his spine, causing him to groan in agony as he felt his spine begin to snap and twist as it re-set itself.
Papyrus heard the sound of bones cracking, which was slightly muffled by Sans's jacket. He looked over and gasped as he realized that the back of Sans's jacket was beginning to bulge. It wasn't long before the fabric tore, revealing the massive spikes that had begun to form along his vertebrae.
"Sans! Oh my gosh! What do I do?!" Papyrus yelped.
"I dunno..." Sans cut off with a wail as another spine-snapping convulsion wracked his body.
Without thinking, Papyrus picked Sans up and set him on the couch. "It's okay, just try to relax. I'll figure something out. I-I'll go find Alphys to see if she can help. Just stay here," Papyrus threw open the door and bolted outside, spreading his wings and taking off as soon as he was outside.
Sans panted heavily as he tried to calm himself. He whimpered as he rolled off the couch, tightly grasping his ribs as pain tore through him once more.
   The pain slowly worked its way into his feet, making him unsure if he would be able to stand if he needed to. He could hear his bones cracking as he felt his feet begin to splinter and shift.
   His feet had begun to push against the toes of his boots, making it feel as though his boots were too small to fit him anymore. His feet felt crushed.
   Sans rolled over onto his side and reached for one of his dark-blue boots. His arms were uncoordinated and he couldn't manage to take the boot off.
   He moaned as pain shot through his legs, completely unaware that the now-sharpened tips of his toes had managed to burst through the toes of his boots.
   Sans could feel his legs starting to grow longer, the bones becoming thicker and stronger. The sound of his bones cracking made him feel nauseous. He wasn't sure how much longer he could fight this.
He felt a strange tingling sensation in the lower portion of his spine, causing him to turn around and look with what little strength he still had. He gasped as he realized that he was beginning to grow a tail.
"P-Papyrus... Somebody... Help me..." Sans whimpered, his voice strained.
As if on-cue, Papyrus burst through the door with Alphys not far behind him. The two monsters rushed to Sans's side, concern washing over them as another wave of pain wracked Sans's body.
A scream forced itself out from between Sans's clenched teeth, causing Alphys to flinch. Sans's voice was gradually becoming more and more animalistic as the seconds ticked by.
   "O-Oh gosh... I don't know how to h-help him..." Alphys stammered.
   Sans let out another groan as pain tore through him. "I dunno, just do something!" he howled, his voice strained.
   Alphys visibly flinched at the skeleton's tone. Sans's skull was beginning to change; his face was stretching out into a long muzzle and large horns were beginning to emerge along the back of his skull.
   There was a loud crack followed by an animalistic scream as Sans's face suddenly split from his nasal cavity to his browline. He stumbled backwards, clawing at his skull wildly as he tried to drive out the pain.
   After several horrifying moments, the transformation ended. Sans was breathing heavily as he stared down at Papyrus and Alphys. There was a feral gleam in his blazing blue eyes.
   Sans growled softly as he took a menacing step towards the two other monsters. Without a second thought, they bolted out of the house. Alphys was scared out of her mind, and for good reason too.
   Sans followed, folding down his wings as he crashed through the door. As soon as he had gotten outside, he spread his light blue feathered wings.
   For the first time, Papyrus noticed that Sans's wings must have grown larger at some point, most likely so that he could still fly. He shuddered at the thought. A huge feral beast rampaging through the monster kingdom... And to make matters worse, he could fly.
   Sans let out a roar as he suddenly lunged for the two other monsters. Papyrus quickly darted towards Alphys, scooping up the smaller monster in his arms before spreading his golden feathered wings and taking flight.
   "P-Papyrus, I can fly on my own," Alphys insisted.
   "But you won't be able to get away from him," Papyrus answered, gesturing towards Sans, who had also taken flight.
Each beat of Sans's massive wings seemed to rupture the air around them. If the huge beast didn't weigh so much, he would have easily caught Papyrus by now.
Alphys screamed as Sans snapped at Papyrus, his jaws nearly clamping down on the other skeleton's large golden wings. Papyrus pulled away last second, his grip on Alphys's smaller body tightening as he did a loop in midair.
The chase continued on for several long minutes. Towards the end, Papyrus found himself growing tired. Each beat of his wings seemed to drain his energy more and more. Sans was catching up.
As a last resort, Papyrus dove towards the ground below. Sans hovered in midair for a split second, clearly not expecting his prey to escape so quickly. Alphys shrieked as Sans soon gave chase once more.
Not having the energy to keep going, Papyrus fell unconscious. His wings were the only thing slowing down his rapid descent. Alphys was outright crying; she was terrified of either outcome of this situation.
Without thinking, Alphys spread her wings and grabbed onto Papyrus, the limp body making it difficult to control her landing. Despite having to carry Papyrus, Alphys managed to stick the landing.
Sans slammed down onto the ground barely a few yards away from the two exhausted monsters. Being that she had more strength, Alphys stood up and looked Sans directly in the eye.
She had seen it in anime: If you looked directly into a beast's eyes and showed no fear, they would leave you alone. Even though she tried to be brave, Alphys was terrified.
Sans took a step towards the winged scientist, a menacing glare in his glowing blue eyes. Alphys felt her soul skip a beat as he took another step. But she held her stare, trying to make it clear that she was the one in charge, not him.
Papyrus slowly opened his eyes and stared up at Alphys. He could tell by the determination in her eyes that she meant business. He kept silent as he watched the scene unfold.
"Y-You st-stay back, you hear m-me?! S-Stay back!" Alphys ordered, her voice shaking uncontrollably.
Sans growled as he slowly took a step back. Alphys stepped forward, a newfound confidence guiding her steps.
"Th-That's right! K-Keep away!" Alphys continued.
Sans roared as he reared up onto his hind legs, slamming his clawed hands back on the ground as he came back down. He squeezed his eyes shut, the feral gleam completely gone when he opened them again.
"Alphys? What're you doing here?" Sans asked.
"You chased us here! You went crazy! You weren't yourself! I was scared out of my mind!" Papyrus rambled, stumbling to his feet as he came to stand beside Alphys.
"Y-Yeah... Papyrus came and t-told me that there was something w-wrong with you... He s-said you were in a lot of pain..." Alphys stammered, looking away sheepishly.
"Huh. I remember being in a lot of pain, but I can't remember anything else..."
"W-Well, you weren't exactly in c-control, so that's to be expected," Alphys pointed out.
"The only thing that matters now is that you are in control now, right?" Papyrus said, walking up to Sans and putting his arms around his front leg.
Sans nodded. "None of you got hurt... right?"
Alphys shook her head. "N-No. But I think we're all l-lucky not to have gotten hurt..."
"Agreed. That would have made everything a whole lot worse, wouldn't it?"
"I'm just glad you're okay!" Papyrus murmured.
"Me too bro," Sans rumbled. "Me too."

A/N: That was so much fun to write! Especially that chase scene! :3
Remember to leave your requests in the comments below! Feel free to suggest any of the more "uncommon aus"!
-Sora