Yes, there might be. You can start by looking on popular fanfiction platforms like Archive of Our Own or FanFiction.net. Just search for 'Xander Spike kidnapped' in their search bars and see what comes up.
Perhaps Xander is kidnapped by Spike because Spike has been cursed or influenced by some dark magic. In this case, he may not be fully in control of his actions. And as the story progresses, Xander could try to find a way to break the spell on Spike while also trying to escape. Another plot could be that a third party hires Spike to kidnap Xander, and Spike reluctantly agrees for some personal gain, like money or a way to get rid of his own enemies. Then Xander and Spike might end up having to work together to face the real threat behind the kidnapping.
Buffy would probably be involved. Since she's the slayer and Xander is her friend, she'd be on a mission to rescue him from Spike. Willow could also be in it. As Xander's best friend and a powerful witch, she'd use her magic to try and find Xander or fight against Spike. And Giles, too. He'd be the one providing knowledge and guidance on how to deal with a vampire like Spike who has kidnapped Xander.
One common theme is the exploration of their unlikely connection. Since they are so different in the original context, fanfiction often focuses on how they bridge that gap.
One common plotline could be that Spike kidnaps Xander for some sort of revenge against Buffy. Maybe he thinks by taking Xander, he can draw Buffy out and have a final showdown with her. Another could be that Spike is under a spell or some dark influence that makes him kidnap Xander, and throughout the fanfiction, Xander tries to break that influence over Spike.
One great Spike Xander slash fanfiction is 'The Unlikely Bond'. It beautifully explores their relationship in a post - apocalyptic setting. The author does an amazing job of delving into their complex emotions.
Since I don't have specific knowledge about 'boypussy xander spike' fanfic, I can only guess. It might have themes related to the relationship between the two characters. Maybe themes of loyalty, or how they face challenges together. It could also potentially have themes of growth, as the characters develop throughout the story.
A likely kidnapper could be a rogue changeling. Changlings are known for their devious ways, and one might think that Spike has some sort of connection to the magic of love that they could use for their own purposes. So they kidnap him to study or exploit that connection.