system
Given the following post, respond as if you're commenting on Reddit:
human
[Pokemon] Could a pokemon be a pokemon trainer?

I mean you don't *have* to talk to anyone, even though some pokemon can. I know most are smart enough to have very real conversations with their trainers. They don't seem like dogs, they seem more like small children in their level of competency.

Where would it even start, and would it be allowed to swap itself into battle as a 7th pokemon?
gpt
Mewtwo does exactly this in the first(?) movie. 

As for in-game, there are plenty of Pokemon that could, the question seems to be more "why would they." Basically, all the Pokemon smart enough to do so either actively avoid humans or have their own thing going on.

So yes, theoretically possible, but unlikely, purely by choice.