The advertisement made it seem like that adorable swine would also go on adventures with Moana and co. But it’s really just the chicken that stow away. I only realized this after I saw the film, and kind of a little disappointed with that false advertisement. Lol.
So what else is new?
I rememeber reading somewhere in the wiki that Pua was supposed to go with Moana in the early version of the film. However, the directors ultimately decided that Moana shouldn't have too many "friends" to make her journey feel more difficult, scary, and uncertain. Pua was too much of a cuddle-buddy and a reminder of home.
Perhaps the only reason why Pua didn't get completely removed from the film was to sell plushies :) The people in my cinema kept "Awww"-ing when the pig was on-screen.