Then again, judging by the Sneasel and Mr Mime ep which featured a grand total of minus nothing of their D/P counterparts - it could probably just be a baby Rattata or something that will belong to Trainer of the Day.
This is different. By the mere nature of how Bonsly was shown with Brock, it has to go to him.
Or, well, I suppose it could go to Max, which would explain what could otherwise be interpreted as Max being one of Brock's Pokémon. But it is automatically assumed Max will always be Pokémon-less regardless of the circumstances...correct?