You can play as an ordinary pokémon. In fact, I don't see why a wild pokémon wouldn't join the trainers, the trained pokémon and the Legendaries. Although if that's the case, I don't see how we could get a diapered wild pokémon.
Just post a short character bio in the thread here:
https://diaperedanime.com/forum/showt...163/index.html.