BtVS created by Joss Whedon and Mutant Enemy. Written for Nerdgirl's "Were-Animals Xander didn't get bitten by" at Twisting the Hellmouth.
Xander was annoyed. He had a hot date in a half hour, yet he was stuck in Willow's dorm room changing Amy's cage. Finished or not, the geas hung on him meant he'd be magically compelled to leave then; as per usual his date was involved in the darker side of magics and she needed a small 'sacrifice'. The moonstone on the necklace he was wearing - a 'gift' from her - was gleaming brightly underneath the folds of his shirt.
As the magic kicked in, sweat poured down Xander's face. Wiping off his brow with one hand, he reached down to retrieve Amy from a shoe box and put her back into her minty-fresh home.
Perhaps he wasn't as careful as he should have been. Maybe Amy the witch turned rat was magic-sensitive and the carpenter suddenly appeared to be a threat. Or else he just suddenly smelled very tasty.
"Ow!" Xander yelled and jammed his bleeding finger in his mouth. He pulled it out just as fast. "Ewww! Rat bites should not get anywhere near my Xander-shaped taste buds!"
Without advance warning, he disappeared in a puff of smoke any stage magician would've been proud of.
Willow, hearing the loud pop, left the bathroom where she'd been scrubbing merrily away. She made it just in time to see a rat scurry under the bed.
After a few minutes of bargaining and borrowed cheese, Willow was able to grab the rat and deposit it into Amy's cage. During the process she noticed that unlike Amy's solid-brown self, this particular black and white hooded rat was male.
A glance in the shoe box confirmed that Amy was still in there and safe. Willow swore something colorful involving jello and dogs, before picking up the phone to call an emergency Scooby meeting.
In one of Sunnydale's more upscale hotels, Serena the enchantress lay waiting. Naked among the satin sheets she always carried with her she basked in the moonlight and waited.
As dawn began to break in the distance, she cursed the spell that should have drawn the Xand-man to her and began to pack.
When the sunlight reached Willow's bed, a very surprised Xander popped back into existence, completely squashing the shoe box he'd been sorted into. Fortunately, he was still fully clothed.
"Okay," he griped, once things had been explained by the gang. "Why didn't you tell me Amy's spell was contagious?"
"It's not, even though last night was a full moon there's no way-" Willow reassured just before the dorm room door burst open and a massive gust of wind pinned all present, including the rodent, up against the far wall.
"That's mine," Serena yelled, eyes blazing as she waved her hand. The moonstone necklace tore its way through Xander's thin shirt and flew to her. "Screw the Hellmouth! I'm going to Seattle!" Sneering, she stormed out.
As she absently pet the recaptured Amy through the usual Scooby discussion of Xander's dating history, Willow put together a brilliant plan.
Just before sunset that night, Xander met Willow in her room. "Good thing the moon should be full enough tonight," he said, frowning. "You absolutely sure she's not in heat?"
"Cross my heart and hope to pass out," Willow announced, unwilling to risk stronger language in Sunnydale. "It's about time." She used assorted blankets and pillows to build a 'wall' around the edges of the bed. "Hop on."
"It's not too bad being a were-rat," he said, light-heartedly. "I've noticed a plus-five bonus to my thief and ninja skills, ignore the reference." He made a face just before night officially fell and he poofed into a rat.
Willow happily picked up a spatula and began to spread a peanut butter and honey mixture onto Amy's paws and fur. "If this doesn't work, missy," Willow said, dropping Amy into the bed/valley with Xander. "You're getting a good bath."
When this did not result in rodent-Xander biting Amy, Willow frowned and telekinetically grabbed hold of Xander's jaws. A nip later and – pop - the still small and furry Xander was sharing the bed with a very naked human female.
Willow swore under her breath and began to fill Amy Madison in on everything that had happened since she'd been trapped in rat form as she filled her arms with fresh clothes. For her part, Amy was taking everything in stride until she realized she was only a were-human and her release from the cage was temporary...
"Oh that's such a beautiful tragedy," Buffy announced the next day at the Magic Box. "He turns into a rat at night, she turns into a rat in the day. It's just like Ladyhawke, only, y'know, without the actual romance."
"Yes, quite," Giles commented. "Except she's only able to turn human three nights a month. Simply unacceptable and potentially damaging to the poor girl's psyche."
"We need a way to keep her permanently shifted," Xander mused. "Any suggestions?"
"Oh, I've seen it... here." Dawn grabbed a book and held it open to a particular page. "This is what we need."
"Well, great." Xander frowned. "Anyone else notice that my attempted-date was wearing a pearl ring with those designs? For all I know she's a were-dragon or something off of Gargoyles."
"You know what that means," Buffy asked the room. "Right?"
"Oooh!" Dawn squealed, raising her hand. "Road trip!"