Yes yes, I don't own anything in the Potterverse, yada yada...
James Potter, however, had other things on his mind that night.
Him and Sirius stole out of the castle beneath James' Invisibility Cloak, with Wormtail the rat leading them down the corridors.
They emerged, unnoticed, from the towering front doors and did their best to keep up with Wormtail, who was already halfway down to the Whomping Willow. As they approached, they saw the heaving mass of branches and leaves freeze, as though caught in time. The two boys scrambled into the entrance before removing the Cloak and stowing their bags in a corner.
Slightly sweaty and out of breath from their hurried and uncomfortable journey, they set off at a jog down the tunnel, not even checking the Marauders map to ensure they hadn't been followed.
Meanwhile, a hunching, sneaky-looking figure squeezed itself from the front doors and hurried down to the Willow, and as the tree began to flail furiously, the figure picked up a branch and pressed a knot in the trunk. The tree froze, just as the figure had been informed it would, and then he dropped into the tunnel and followed it, completely unaware as to what surprises lay ahead.
Severus Snape followed the tunnel and eventually slowed down to a sneaking crouch, until an incredibly bizarre scene met his eyes.
For the boarded up room did not reveal four teenage boys, as he had expected, but two creatures he took a moment to recognize.
The nearest to Severus was a great, handsome stag, whose admirable antlers would have been ideal for brewing many potions.
The second creature, Snape realized with a jolt, was no Animagus like the other, but a werewolf, fully grown and transformed.
It seemed that at the very moment Snape noticed the werewolf and began to panic and back away, the werewolf realized he was there. Directing his long, pointy nose towards the doorway, he gave a sniff and smelled fear emanating from the entrance. The werewolf gave an ear-splitting, bone-chilling howl. The stag caught on; it poked its head into the cave, and saw Snape standing there, white-faced and shaking.
Severus heard a crash and two more creatures came scampering in: a grey rat and a huge, shaggy, black dog. All four caught sight of Severus.
The werewolf lunged for him, but the stag got there just in time; it caught the creature in its antlers and threwit across the room.
Severus watched, to his amazement, as the stag morphed into James Potter, who turned to Snape and yelled, "RUN!" before returning to his stag form.
Snape didn't need telling twice. He started up the tunnel faster than a broomstick, emerged from the Willow (which he promptly Immobilized), and sprinted up to the castle, not daring to look back.