The Saturn Rider is an item which originates in the Earthbound/Mother series. It is actually a Mr. Saturn-shaped coffee table which can run, and can also be ridden. It has appeared outside of the Mother series in Super Smash Bros. Brawl and in the latest entry for 3DS/Wii U.
In the Mother/Earthbound series
The first appearance of this object was as a normal coffee table in Earthbound on the SNES (Otherwise known as Mother 2) in on of the houses in Saturn Valley. It looks exactly as it does later, though here it is simply a table and cannot move. In Mother 3, it appears in Chapter 7 where it leads the main team to Saturn Valley.
In Super Smash Bros.
The first appearance of the Saturn Rider in Smash Bros. was in Super Smash Bros. Brawl, with the debut of Lucas. It only serves as a cameo during the start of any battle with Lucas, as Lucas will ride the Saturn Rider onto the stage, get off, and then the Saturn Rider will run away. Since Lucas also appears as a DLC character for Super Smash Bros. for Wii U/3DS, it appears alongside him again in his introductions.