Finally, some explanation on why such a cute dog is in beat-em up game! Today, Nintendo told us about a new kind of Pokeball like item called Assist Trophies. Like Pokeballs, you have no idea who’s inside until you use the item. And you can’t throw the item at other players.
It would seem like Assist Trophies can be bad or good. 
With the Nintendogs, it would seem they act like a Porygon 2 and a Golden. We assume that when it moves it’s paws up and down, there is a potential for hurting other players, yet it really seems to do nothing very helpful as it just blocks the entire screen.
Other than Nintendogs, Nintendo revealed a Hammer Bro as one of the many Brawl Assist Trophy characters (they say that characters who couldn’t make it as a playable character were made as Assist Trophy characters). The Hammer Bro can attack other players by chucking hammers at them at very close range.
Unfortunately, you can’t control your Assist Trophy character, which might be good or bad in some cases.