Little Red Riding Hood
There was the Wolf, and there was Remus, and he did his best to keep them separate. Remus was monogamous; he had lost the love of his life when Sirius died. The Wolf took love where he found it and he found it often. Remus was a good man, one who played by the rules. The Wolf did as he pleased and it pleased him to do wrong. They shared a body, and they shared memories.
Remus remembered the way Sirius closed his eyes when he climaxed, the sweetness of his smile after. The Wolf remembered that Sirius liked to draw blood, that he disliked lubrication. Remus remembered running in wolf form beside Sirius in canine form. The Wolf remembered the dog crushing their throat in its jaws as its weight crushed them against the floor of the Forbidden Forest. Remus remembered that Sirius could be impatient, imprudent, that he did things for love a wise man would not have done; the Wolf remembered being raped.
Remus remembered how beautiful Lily Evans had been, radiant even, as she fed the infant Harry. The Wolf remembered how her breasts had grown, that her nipples were the size of saucers. Remus remembered that she was lonely when James was at work and the Wolf remembered that she was ravenous. Remus blamed himself for seducing his best friend's wife but the Wolf had smelled how wet she was when she noticed heir arousal. Remus was sorry but the Wolf had enjoyed taking her from behind because it was too soon after the baby for the other way. Remus never noticed Harry watching them.
Remus had always loved James the best, though he would never have admitted it; he had been flattered when James came to him. The Wolf had only been surprised at what it was James wanted from them. Remus had been afraid he'd hurt his friend, coming into him in wolf form while James was human. The Wolf had known James wanted to be hurt, that they were not the first large animals to force themselves into that tight entrance.
Minerva McGonagall put Remus off women for good, after an incident at a Hogwarts reunion in the late 1980s. The Wolf rather liked being disciplined, though neither of them really had a taste for broomsticks. Remus had done as he was told and the Wolf had been naughty, and the professor had rammed the toy into both of them with a force that made them come.
Remus had wanted to steer well clear of Lucius Malfoy but the Wolf had recognized a fellow predator and they had spent an intriguing night watching Severus Snape crawl and scream on the rack. The Wolf had noticed Severus was tighter than Lucius, tighter even than James, tight as a virgin. He did not get hard at all that night, though Lucius and the Wolf plied him with aphrodisiacs and plowed him with erections that never failed. All of this had done little to foster inter-House relations, of course, when Remus was granted a teaching position.
At Hogwarts Remus behaved with the utmost propriety but the Wolf spanked Ginny Weasley and taught Draco Malfoy a thing or two about discipline and Fred and George Weasley a thing or two about practical jokes. There was Zach Smith, who asked questions throughout, and Flitwick, hung like a gnat, who cried after. There was even, memorably, Albus Dumbledore, who offered the Wolf sweets and wanted the house elves to watch.
For a long time there was Remus and there was the Wolf, and they were mostly separate. After Sirius died, though, Remus let the Wolf out to play more often. Eventually people forgot all about the mild- mannered, shabbily dressed schoolteacher. Remus became the Wolf, and the Wolf was a very bad man indeed--the sort who ate little girls he barely knew.