The First 15 Minutes of Mega Man Anniversary Collection: Mega Man III (GameCube)

Copied from MobyGames:

Mega Man 3 is a traditional side-scrolling platformer: you jump from brick to brick while avoiding pits and shooting robots. It's quite similar, in all respects, to other Mega Man games; however, this is the first time that you get a sidekick. It's also the first time you're allowed to slide instead of merely walking.

Rush is your dog who helps out by offering other modes of transportation when you need it. He starts out with the 'Rush Coil' ability to launch you to the top of the screen like a springboard; later he acquires the 'Rush Jet' ability to turn into a guided, floating platform to carry you wherever you need to go; and he also gets the 'Rush Marine' ability which is a submarine that works just like the jet except underwater and with the added ability to hop around out of the water (you can actually jump while on the jet but the jet continues to hover beneath you whereas you stay inside the submarine). All of these abilities function just like boss weapons.