Sorry this one has taken so long! I kinda lost inspiration to write over the past few days. :/
Anyway, this is DanceTale Sans and it was requested by lokiplayz
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   Sans stared at the ground as he walked through the woods. He was unaware that his eye was glowing from underneath his hood, which he always kept pulled over his head.
He sighed as he stuffed his hands in his pockets. He had a lot on his mind, but nothing in his thoughts actually made sense.
His thoughts were jumbled... A bunch of nonsense that he couldn't piece together. He gritted his teeth in frustration.
He growled quietly under his breath. He wasn't sure if he would actually be able to dance like he normally did when he went out into the woods due to the dull ache he felt in his bones.
   Maybe he was just tired??
   Sans shook his head, as if to clear his thoughts. He was probably just having one of those "off" days.
After several minutes of walking, Sans made it to the clearing. By that point, he had lost all of his energy. His bones felt... heavy... weak...
He collapsed in the snow, his hood all but hiding the unmistakable blue flare from his left eye socket. He could feel sweat dripping from his skull as he panted softly.
   "Deep... breaths... Come on dude... It'll pass... It has to..." Sans mumbled, curling up into a ball as the ache in his bones began to grow worse.
He clutched a hand to his sternum, faintly aware of a burning sensation that was just beginning in his chest. His soul was pounding loudly in his ribcage, making it feel like his bones were rattling with each beat.
If he'd had the strength to look down, he would have seen a bright blue light pulsing within his ribcage, the faint glow slightly dimmed by the fabric of his shirt.
The heavy throbbing of his soul was becoming too much for him to handle. It was starting to make his chest hurt even more than it already did.
The burning pain that had started in his chest was beginning to spread throughout the rest of his body. He felt like he was about to burst into flames at any given moment.
The burning ache in his body was starting to make his bones cramp. He rolled over onto his side, moaning as a wave of pain came over him.
   "Oh god... Why does it hurt so much?" Sans panted as he dug his fingertips into the sleeves of his jacket.
   He was in too much pain to notice that the fabric of his jacket was beginning to tear where his fingertips were. When he finally looked at his hands, he couldn't help but gasp in dismay.
   The tips of his fingers had grown sharper. Before Sans could react, pain tore through his body, causing him to let out an agonized groan as he began to arch his back, his spine starting to re-set itself with a series of dry snaps.
   He rolled over onto his ribs, gasping as pain overtook him once more. He felt an unusual weight beginning to form along his back as spikes began to form along his vertebrae.
   Eventually, the massive spikes began to push through the fabric of Sans's jacket, causing the back of hoodie to bulge before tearing down the length of his spine.
He let out a pained grunt as he felt his feet begin to change as well. He could hear the sound of his bones beginning to creak, as if pressurized.
Sans let out a wail of agony as the joints in his legs and feet suddenly began to crack back on themselves, sending unbearable waves of pain through the lower half of his body. His feet were beginning to elongate as the tips of his toes grew sharper and eventually burst from the toes of his sneakers.
Sans felt a strange tingling sensation in the lower portion of his spine as he began to grow a tail. The agony was too much for him to handle.
   He was scared.
   Sans could feel his skull beginning to change, his face elongating into a muzzle as several bumps emerged along the back of his head. It wasn't long before the small bumps on the back of his skull erupted into massive horns.
   He could feel his teeth growing sharper as his skull suddenly split from his nose socket to his browline, forcing a scream out from between his clenched teeth.
   After what seemed like forever, the transformation ended. Sans just laid there on his side, fearing that if he moved, another wave of pain would be sent through his body.
Eventually, he got up and shook his body like a dog would, his bones rattling as he moved. He looked around, his gaze resting on a dark figure darting between the shadows of the trees.
It was the human. Sans started running towards them, but he stopped short as a sudden urge to attack came over him. His vision darkened as he lost control of himself.

(POV Change)
Frisk stopped dead in her tracks as she heard a rustling sound behind her. She spun around, instantly spotting Sans's glowing blue eye in the shadows.
"Sans!" Frisk shouted excitedly, waving to the skeleton.
But she stopped herself in her tracks. Why was Sans's eye glowing? Was he mad about something? Also, why did it seem like he was... taller than usual??
   She shuddered as she realized that he was a lot taller than usual. The human flinched as she heard a low growl from Sans. It didn't even sound like him!
   Frisk gasped as he stepped out of the shadows. This wasn't her Sans! This was a huge beast!
   She backed away, her eyes wide with terror as the massive creature took a step towards her. His single glowing eye blazed with an icy-blue flame, the light in his other eye not in existence.
It was like staring into two different abysses. One was ice-blue and the other was pitch-black. Both were so cold... so unfeeling...
   His left eye gave off a terrifying feral gleam, making Frisk shiver at the sight.
   "Sans..." Frisk trailed off as the beast took another step.
   A low growl was the only response the huge creature gave. He continued to advance on the small human, each step making Frisk's heart pound harder and harder.
   With a loud roar, the beast threw himself at the human. Frisk screamed as she darted away, narrowly avoiding the attack. Snow flew through the air as the beast hit the ground with a thud.
   He thrashed wildly, trying to get back on his feet. Without a second thought, Frisk ran. She made a mad-dash to Snowdin Town, hoping that she could find a place to hide there. Perhaps she could find Papyrus! He should know what to do!
The beast snarled as he gave chase, his footsteps shaking the ground ever-so-slightly. Frisk tried to outrun the massive creature, but he was gaining on her rapidly.
   She wasn't going to make it. He would catch her before she made it to the town. The beast snapped, his jaws barely missing the seam of Frisk's tutu.
   She yelped as she felt his fangs skim the edge of her skirt. She looked back just as he snapped again, his jaws clamping down on her frilly pink skirt.
She couldn't react fast enough. Before she could even do anything, the huge beast had her in his jaws and was tossing her up into the air. Frisk screamed, her heart pounding so hard that she was certain that it would burst out of her chest at any second.
Just before she hit the ground, the beast caught her by the skirt of her tutu with a single claw. He held her up to eye level, the scent of ozone flooding through Frisk's senses as the massive creature brought her closer to his muzzle.
Blue smoke poured from his jaws as he let out a low snarl. The light from his left eye was almost blinding; it almost looked like there was a mini super nova in his eye.
Frisk gulped. This was it. This was where she died.
Then, much to her surprise, Frisk felt herself being lowered back down to the ground. She looked up, seeing a guilty look in the beast's eyes.
He stepped away, purposely avoiding eye contact with the small child that he'd nearly killed.
"I'm so sorry kiddo... I didn't mean to hurt you..." the beast rumbled.
Frisk's attention snapped to the huge creature. That voice... It was oddly familiar...
Despite the animalistic tone to his voice, Frisk could detect a hint of the voice she knew all too well.
Deep inside, this beast was still her Sans. He might have changed on the outside, but he was still his old self on the inside.
   "Sans... It's okay..." Frisk murmured, walking up to the skeletal beast and hugging his leg.
   "No, I... I fucked up... I'm sorry I hurt you..." Sans mumbled, tears forming in the corners of his eyes.
   "Hey," Frisk said, flicking Sans's leg to get his attention.
   Sans looked down, a confused look on his face.
   "Don't be sorry for what you did. I'm not hurt. And even if I was, I wouldn't be angry. You couldn't control it. I don't blame you," Frisk insisted, her eyes gleaming with determination.
   Sans sniffled slightly as he looked back at the small human. "Really? After what just happened, you still forgive me?"
   "Of course. You're my friend. And nothing is gonna change that," Frisk replied reassuringly.
   Sans sighed. "Thanks kid. Thanks for believing in me."
   Frisk giggled softly as she stroked Sans's leg gently. "You're welcome."

A/N: Gosh, once I finally got around to this one, it was fun!
Also, I was wondering if you guys would mind me doing a oneshot with Lust Sans. It wouldn't have any ACTUAL smut, but it would be implied.
I want to do it, but I don't want you guys being... uncomfortable with that particular au.
Anyways, tell me what you think in the comments below! Don't forget to leave your other requests as well!
-Sora