From the Super Mario Wiki
Not to be confused with "Thunder Cloud".
The Rain Cloud is an item appearing in Mario Kart Arcade GP and its sequel, Mario Kart Arcade GP 2. The effect of the item is that a cloud will appear above the opponent's head and make the kart harder to control. The item is heavily based on the Thunder Cloud in a few ways. The item is unlocked sometimes by winning the Mario Highway race in Grand Prix mode for the first time.