"Oh, God!" Sirius yelled, instinctively shoving his best friend James Potter backwards and away from the mousey-haired boy. His form began to hideously contort, nose turning to snout, hands turning to paws, until he was Padfoot, his animagus. Growling menacingly, he watched through amber canine eyes as the mousey-haired boy contorted too, though into something far larger and more dangerous.
The Werewolf looked confused for just a moment, wiping drool from its grey lips and glancing around the room as though frightened. Then it spotted James. It advanced slowly, all traces of Remus Lupin absent from the bloodthirsty grin and lunged forwards, only to be tackled away with the full force of Padfoot.
James scrambled and dropped to his knees, crawling beneath a common room table and drawing out his wand as a precaution.
"He shouldn't be like this tonight!" James hissed, glancing out the window to the full moon. They had been careful to closely study the astrology charts and each of them knew exactly when the full moon was coming. With no time to dwell on this, however, he focused on finding a good place to transform. Padfoot was struggling with the Werewolf, suffering serious gashes in his side. There wasn't enough time to wait for Moony to transform back into a human, so they would have to lead him outdoors. He was too dangerous inside Hogwarts, there were too many people.
"Wouldn't matter so much if we sacrificed Snivelling Snape" Potter thought with a cheeky grin. He pointed his wand at the table above him and whispered: "Wingardium Leviosa!" The table hovered a few inches off the floor and he elbow-crawled towards the inside of the portrait of the fat lady. Once he had reached it, he threw the table off himself with his wand, causing the Werewolf to stop wrestling with Sirius and snap it's head towards where the noise had come from.
James transformed into the magnificent fawn coloured Stag that was his own animagus and rammed the portrait wide open. Sirius pinned his teeth into Werewolf Lupin's hairy arm, causing the Werewolf to yelp as it tried bolting towards the very delicious-looking young Stag. Padfoot waited a few seconds for Prongs to clear the top of the stairs and make it half-way down, then for the stairs to move and join another path, the flight down to the main hall corridor. He released the Werewolf, who took a swipe at him, knocking him sprawled onto the red and gold rug by the settees and watched it sprint, bounding with pink tongue almost flying behind it, after James. He licked a small scratch on his front paw and promptly lifted himself back onto all-fours to pursue in the chase.
James was a fast Stag, having trained up his human body to be the best Seeker he could. He should be able to keep away from the Werewolf long enough to reach the Shrieking Shack, Sirius was sure of that, but what if the Werewolf lost interest and wandered off into the Forbidden Forest? Lupin wouldn't survive the night with the creatures lurking in there and then they would all be devastated. Especially James, whose duty it was to check the weather and whether there would be a full moon, though Sirius, too, was sure that the full moon should not be out this time of the month. He shook his charcoal mane and focused on running, his canine heart beating faster than his human heart could probably withstand. He leaped down the staircase and skidded along the corridor, just seeing Lupin's werewolf tail disappear round the corner and down the stairs towards the main hall.
"Man these injuries are gonna be a hard one to explain to McGonagall tomorrow" He thought, sniffing with irritation. Sirius bounded after James and had to slide to a halt on the newly-polished marble of the Entrance to Hogwarts as he morphed back into his human form, withdrew his wand and whispered "Alohamora!". A blue spell flew towards the great oak doors and unexpectedly… rebounded. Sirius, already back in his dog form, cursed sub-vocally. The doors always opened with Alohamora!
The Stag James glanced back, a look of mixed terror and determination in his eyes and braced himself, lowering his strong horns. Sirius transformed back into a, rather naked, human and wolf-whistled at Lupin. The Werewolf snapped his head back. They weren't the brightest of creatures, luckily.
"Oi, Moony. Have some of this tasty flesh!" Sirius taunted, pulling at his own stomach and wiggling his backside at the wolf, who without hesitation, changed course. Padfoot once again appeared, tackling the wolf to the ground and struggling with it so that James could ram the heavy doors open.
James rammed and rammed, focusing the blows of his horns on the lock and eventually it opened. Obviously the teachers had not felt the need to protect against brute force, only magical spells. James scoffed, mostly amused at this notion as the door creaked open slightly. He pushed it open and bolted outside, on the path for the Shrieking Shack. Sirius again waited just enough time and then let the Werewolf, whose attention had once again snapped back to the Stag after the naked Sirius had 'mysteriously' been taken off the menu, go after it.
After a long and tiring chase, the three reached the Shrieking Shack and entered, with Sirius trapping the Werewolf and James transforming back into his human form to capture the Werewolf by chaining its arm to the wall and placing strengthening spells on the handcuff, while keeping a fair distance away, so as not to have their heads ripped off.
Sirius remained as Padfoot for most of the night, warding the Werewolf off by growling from the opposite corner of the room, stopping only to try and cleanse some wounds with his floppy pink tongue. James, however, stayed human, wondering aloud how he could have messed up with his astrology, causing Padfoot to eventually shoot him a warning glance that said "Shut up!".
After what seemed like weeks, the moon was replaced by the morning sun and the Werewolf yelped helplessly as it's shape contorted and twisted back into the very pale, shaking Remus Lupin. James, who had gotten dressed earlier that morning with the spare clothes they kept at the shack, threw some over to Sirius and then hurried over to Lupin.
The Potter boy looked down at his shivering friend with pity and used that tone of voice they always reserved for the morning after a full moon. "Alright, Moony?" He said with care.
"Y-yes… what happened?" Lupin asked, bewildered.
"You transformed" Sirius informed him when James couldn't give an answer.
"I transformed?" Remus asked, blinking. "B-but, there wasn't supposed to be a f-full moon last night!"
"Tell us about it" Sirius moaned, pulling his cloak around his shoulders. "You went all werewolf on us in the common room."
Remus went even paler at this idea "I-I didn't…?"
"Kill anyone? No." Sirius grinned "Gave us quite the chase up here, though"
Remus gave a nervous giggle.
"You killed the common room furniture! That counts!" James commented, perking up.
"Yeah," Sirius sighed, scratching his head "We'll be getting detentions for months for that, I'm sure."
Lupin gasped.
"If they find out who did it!" He added, with a cheeky wink.