Theory by Snivy1012

Ash's Snivy was Team Plasma's before being Ash's Pokémon.

Well Iris and Cilan speculated that Snivy abandoned her trainer, as she deemed unworthy and Snivy is clever, and she is at a very high level and has some battle skills, so it might have been from some kind of Team that she didn't like their ways and abandoned them.

Thanks for the theory 😉