Koopa Clown Car
The Koopa Clown Car (sometimes spelled as the Koopa Klown Kar) is one of Bowser's methods of travel. It is the shape of a bowl, big enough to hold Bowser and perhaps one other human, with a small propeller on the bottom that propels the whole thing up in the air. There are no obvious controls; Bowser seems to be able to control it (and call it from a distance) mentally. The body has a smiling clown face on it; why it's not a Bowser face or something scarier instead is unknown.
The Koopa Clown Car's earliest appearance was the final boss battle of Super Mario World, in which Mario had to defeat Bowser as he flew around the stage in the machine. It held Big Steelies and Mecha-Koopas to use as projectiles, as well as Bowser and the captive Princess Peach. The Clown Car appears in other games as a form of travel for Bowser, such as in Mario & Luigi: Superstar Saga. Recently, in New Super Mario Bros. Wii, Bowser Jr. has his own, smaller Koopa Clown Car and uses it to battle Mario. He also uses his father's Koopa Clown Car much later in the game.
 Trophy description
Bowser's single-seat, personal airship doesn't exactly handle like a dream, but it wasn't designed to be sporty. The Koopa Clown Car was made to carry Bowser's huge weight, not to mention a logic-defying arsenal of massive bowling balls. In Super Mario World, Mario busted the Koopa Clown Car by pelting it with Mecha Koopas.
Initial speculation theorized that the Koopa Clown Car would be Bowser's entrance in Brawl. However, it turned out that it was only used as a plot device in the Subspace Emissary, allowing Bowser to get out of tight spots without having to deal with any of the heroes. Unlike in Melee, there is no trophy of the Koopa Clown Car.