One possible scenario in a fanfiction could be that Sylveon is in a happy mood and its tail wagging is a sign of its excitement. Maybe it has just found a really delicious treat or it's about to play with its trainer.
In a fanfiction, Sylveon's tail wag might also be a form of communication. For example, it could be wagging its tail to signal to other Pokemon that it means no harm. It could be in a new environment and trying to make friends, and the tail wag is its friendly gesture.
If you want the clearest chapter reference, go to Chapter 231. Early on, Huo Xuan felt a burst of secret delight in her heart.; later, the chapter leans on Huo getting angrier, she became even more adept at playing the innocent....
The best chapter pointer here is Chapter 273, ""Justice Will Eventually Triumph Over Evil". It opens with Jiang Li, who was reading a book, suddenly jumped in fright. and closes on The two walked side by side in silence on an even quieter path..
The best chapter pointer here is Chapter 238, "The Guests Were Dumbfounded, The Audience Was Excited". It opens with Jiang Li was actually first on all three leaderboards?! and closes on And, to Zhou Xu's alarm, they did find a problem..
The best chapter pointer here is Chapter 268, "I'm Just Jealous of Jiang Li, I'm Annoyed by Her!". It opens with To ensure the company's successful listing, Shen Lan even went so far as... and closes on Jiang Li has nothing to do with Director Sun!" "Because she is my niece!"....
Look to Chapter 251 for the clearest answer. It begins with If Du Qinglian was somewhat angry just a moment ago, now she was simply at..., keeps building through How had this child changed so much after coming out of the mountains?, and lands on The moment the connection was successful, before.
The best chapter pointer here is Chapter 223, "Pigpen Live Stream". It opens with So, for the first time, Jiang Yan, who had always styled himself as the... and closes on The damned Jiang Li was tricking him again!.
Chapter 237 is the strongest chapter-level answer because it starts from It seemed as if, believing the spilled water couldn't be recollected, Ah... and follows through on The netizens responded in chorus, and no one insisted on forcing Jiang Li....
Look to Chapter 264 for the clearest answer. It begins with Jiang Li first reassured her mother, then opened the chat box with Shen..., keeps building through She had just sent a message when Shen Lan's video call came through., and lands on [Do your faces hurt, you rumormongers?.
A practical answer is Chapter 215, because it turns the idea into a concrete scene. The chapter opens with Jiang Li let out a light sigh and tried to adjust her posture., adds pressure through Not only did their free time increase, but they also felt less restless,..., and ends on He originally.
Chapter 219 is the strongest chapter-level answer because it starts from Qi Tianyu was somewhat freaking out. and follows through on Having any yield at all was already the show giving them face..