"I once saw a boggart make that very mistake-- tried to frighten two people at once and turned himself into half a slug..." Remus Lupin, Prisoner of Azkaban
Full to bursting with second and third helpings of pudding, James, Sirius and Remus slowly made their way back to Gryffindor Tower. The first term of their first year at Hogwarts was over and they were heading home tomorrow the for Christmas holidays. Peter had gone up before them to try to dig out his last pack of Exploding Snap cards, as he was the only one of them with any left. They planned to spend this last night in the Common Room playing games until they couldn't stay awake any longer. Holidays were for sleep; the end of term was for playing.
They passed through the portrait hole and into the Common Room, but Peter still hadn't come back down. The three decided to go upstairs and help him look for the cards, and maybe, as a concession to Remus's insistence that packing was best not left for the last minute, tossing a few things into their trunks along the way. Halfway up the stairs they were nearly bowled over by a panic stricken, wide eyed Peter who was on his way down stairs with a speed they had not thought possible for him. "B-b-boggart! In my trunk--I'm getting Professor McGonagall--"
Peter continued to stumble down the stairs, leaving the other three in a quandary. None of them wanted to face the Boggart waiting in their dormitory, but neither did they want to look frightened in front of their friends. Each of them conjured up clear mental images of what shape the Boggart would be likely to take for them, and were confronted with the memories that caused their fear.
James remembered the last day before he'd left for Hogwarts. He'd spent the morning practicing Quidditch on his new broom and imagining himself as the star of the Gryffindor team. His mum had left his lunch by the broomshed, professing no desire for, "half of the outdoors to be brought inside," on his shoes, but most likely taking one last chance to indulge her son that summer. As he was eating, he slowly became aware that an itch had been bothering him for awhile and he sat up just enough to be able to reach his calf for a scratch, barely paying attention. But when his fingers made contact with something squishy and slimy he sat bolt upright. A slug as long as his hand and fat as a sausage was attached to his leg. He tried to brush it off, but it wouldn't detach and a small trickle of blood started to pool in the grass under his leg. He hadn't been aware that he was screaming until his parents had come racing out of the house.
His mum had kept calm and sprayed the Flesh-Eating Slug with a potion that had made it shrivel up and fall off his leg. "Lucky you noticed it when you did. It's already made a gouge the size of a Galleon," murmured his dad. The sight of the gaping wound that the Slug had nibbled away almost made him faint, but he did his best to summon up some Gryffindor courage and act as if it barely hurt. He'd been patched up and healed quickly, but since then he kept a keen eye open while sitting in the grass. He'd also had a fair few dreams about dozens of Flesh-Eating Slugs crawling on him or coming out of odd places like the shower tap or his school bag, and he was afraid that his courage might falter, even in front of his friends, if the Boggart became a giant Slug.
Adopting a confident swagger that belied his inner quaking, Sirius was the first to actually start back up the stairs. His childhood home had not been pleasant to live in, and Boggarts had not been uncommon in the dark corners of Grimmauld Place. Over the years, his encounters with Boggarts had presented him with a variety of images, reflecting whatever he'd found frightening at that age. But he'd been confronted with the same basic shape for the last four years and he knew exactly why. Every time he went down the main stairs, he had to pass a creepy row of mounted House Elf heads. The principle of beheading an ancient House Elf hadn't bothered him much; indeed, he'd much rather Kreacher be there with the others instead of skulking around reminding him of how much of a disappointment he was to his parents.
It was unnerving to see those heads all in a row, silent and unmoving. As a young child, he'd always feared that as soon as he passed them, they turned to watch him, and he would spin around lightning fast to catch them at it. He'd had bad dreams about a headless House Elf corpse stumbling around his bedroom looking for its head. His last encounter with a Boggart was when he was packing up for school. He'd opened up an old trunk his mother had sent up to his room to toss in his things, and a six-foot tall, partially decomposed corpse had stepped towards him as if trying to take his head to replace the one that it had lost. He still wondered if his mother had known that the trunk was home to a Boggart--he wouldn't put it past her to consider it amusing to have one last chance to torment him before he left for Hogwarts.
This was the sort of thing that had given Remus nightmares since he received his acceptance letter: his private fears and problems made public. James and Sirius were nervous too, but all they were worried about was looking foolish in front of their friends. Remus had secrets that he needed to hide if he wanted to keep his friends. He'd been afraid from the start that he wouldn't last more than a few months without getting caught, and his throat tightened as he realized that this might well be that moment.
Remus was fairly certain that the other boys wouldn't think it all that unusual for a Boggart to take on the shape of a werewolf. After all, wizarding children grew up with warnings about going out at night at the full moon, and most were afraid of werewolves, banshees, hags and the like. But if the werewolf-Boggart didn't act in a normal way, or had, as he suddenly feared, the body of a regular werewolf with his own human face... He simply wasn't sure how to go about explaining it away in a tone that would be casual enough to convince them that he was telling the truth.
At the top of the stairs, all three boys paused slightly and were secretly relieved to notice the hesitation of the others. Sirius and James walked in the door together while Remus, still wishing there was a way to not go in without losing face, hung back a bit.
The lid of Peter's trunk was slightly ajar, and the Boggart, sensing James first, turned into the largest Flesh-Eating Slug they'd ever seen. Then, the slug turned towards Sirius and quickly changed into a seven-foot tall rotting, headless corpse with outstretched arms. It began to stumble towards them, but paused in confusion as it once again sensed James. Wavering a bit, the Boggart finally turned into a large, headless slug.
Their relief may have created an exaggerated sense of euphoria, but James and Sirius were sure of one thing--they had never laughed so hard.
Weakened and angered by their laughter, the Boggart finally sensed Remus, and taking one last try at being frightening, turned into a full, luminous moon. Just as the awkwardness of James and Sirius's faded laughter started to become unbearable, Professor McGonagall walked in trailed by an out of breath Peter. "Riddikulus," she commanded, and the Boggart quickly turned into something they couldn't quite make out, and took refuge in a small trunk she was carrying. As she left the room, the boys turned to look at Remus, who was slumped against the wall, mouth open, his face an unhealthy, pasty white. He backed out of the door, casually saying, "What do you know? Guess I am still afraid of the dark. Professor! Let me help you with that trunk..." as he fled the room.
Neither James nor Sirius said anything further that night about Remus's Boggart. Privately, though, they both felt as if they had just found the last missing piece of the puzzle.