Another baby bones sans and dadster oneshot. This one was actually requested by izz2772
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Sans woke up, immediately becoming aware of a dull ache in his chest. He slowly got out of bed and stumbled across the room. Clearly, his stumbling had woken up his brother, because before he knew it, the younger skeleton was clinging onto his leg.
"Where is Sans going?" Papyrus asked.
"N-Nowhere. I'm just feeling a little... off today... It's fine..."
Papyrus tilted his head to the side. "What's wrong?"
Sans winced as he felt the dull ache in his chest grow worse. That was when he realized what was going on. He needed to get away, and fast.
"Pap, please. Let me go. I've gotta go do something," Sans tried to reason with his brother, desperately trying to yank the younger skeleton off of his leg.
   Sweat began to form on Sans's forehead as he fought to free himself from Papyrus's surprisingly strong grip.
"Let me come with you big bro!" Papyrus insisted, his grip tightening on Sans's leg.
Sans grunted as he tried to throw the smaller skeleton off. He could feel the pain in his chest beginning to spread. He didn't have much time.
Before he could even react, a surge of pain shot through him, causing him to double over while clutching at his chest.
"Big bro? What's wrong?" Papyrus asked, his eyes flashing with concern.
Sans muffled a grunt of pain with his sleeve, trying like hell to hide his agony. Papyrus's grip had loosened on Sans's leg, giving him a chance to escape. Without thinking, Sans shoved his little brother to the ground and ran out of the room.
He had barely made it a few feet before he collapsed, another surge of pain through his body making it impossible to get back up.
Before he knew it, Papyrus was at his side. He was trying to speak to him, but Sans couldn't hear what the younger skeleton was saying. He was too focused on the pain.
He curled up into a ball, sobbing quietly as the pain grew unbearable.
"Big bro! What's happening to you?!" Papyrus gasped as he began to notice that Sans's fingertips were growing sharper.
Sans groaned as a spine-snapping convulsion wracked his body. "B-Bro... Y-You need to go get Dad..."
"Why? What's going on?"
Sans opened his mouth to respond, but he cut off with a wail of agony. "J-Just do it... Please Pap..."
Papyrus hesitated, but eventually nodded before running off to find Gaster.
"Please hurry..." Sans murmured, his voice hoarse.
He let out another cry of pain as he felt his spine begin to snap and twist, re-setting itself as spikes began to emerge along his vertebrae.
   After what seemed like ages, Papyrus came back, Gaster not far behind him. Through the fog of pain, Sans felt himself being rolled over onto his back. He looked up, just barely managing to make out the face of his father staring down at him.
   "D-Dad..." Sans cut off with a whimper.
   "It's okay Sans. Don't try to fight it," Gaster murmured.
   "But it's gonna hurt..."
   "It'll hurt more if you try to fight it. Trust me," Gaster insisted.
   Sans screamed in agony as pain shot through him once more. He kicked out his legs, nearly clouting Gaster on the face as he did so.
   Gaster felt something smack against his arm, causing him to look down. He could see that a tail was just beginning to form from the lower portion of Sans's spine.
   Gaster rubbed Sans's skull gently, trying to calm the poor child down. Papyrus watched in silent horror as Sans's feet began to elongate and the tips of his toes grew sharper.
   His face was beginning to stretch out into a long muzzle as several horns erupted from the back of his skull. Papyrus flinched as Sans's face split from his nose socket to his browline with a loud crack. A groan forced itself out from between Sans's clenched teeth, which were gradually growing into sharp fangs.
   After several horrifying minutes, the transformation ended. Papyrus stared into the eyes of his older brother, expecting to see some sort of feral gleam in his eyes.
   Sans was breathing heavily as he stared back at his brother. The two brothers looked at each other for the longest time, neither one daring to move.
   As Papyrus stared into the icy-blue depths of his brother's eyes, he realized that he wasn't a threat. The more he looked at him, Sans almost looked like a large dog. He grinned as he walked over to his older brother and hugged his skull.
   "You look like a big puppy!" Papyrus squealed, his eyes lighting up joyfully.
   "Y'know Pap? You're right. I do look like a big puppy, don't I?" Sans chuckled.
   Sans put his arms around his brother, trying his best to give the smaller skeleton a hug despite not being used to controlling his new form.
   Gaster smiled down at his two sons, closing his eyes as he listened to their conversation. Surprisingly, listening to the two boys talk was really calming.
   He was so happy to be the two brothers' father. He wouldn't give up one second with them, even if he really had to.
   He loved them, no matter how much they bickered. They were brothers, after all. It's what brothers do, right?

A/N: Awww! Another fluffy one! :3
Make sure to leave your requests in the comments below!
-Sora