Thundercloud

The Thundercloud is an item that was introduced in Mario Kart Wii, and can be obtained from item boxes scattered throughout tracks. It was one of the three items that was debuted in this game, but has not made a reappearance since.

When obtained by the driver, it delivers them a brief period of continuous boosting. However, it must be transferred onto another driver within the short limit by colliding onto them, as when it disappears it will electrocute and shrink the unfortunate driver, much like the effects of the standard lightning. Therefore, the item pretty much serves as a double-edged sword - it can benefit the driver by harming an opponent, or harm the driver themselves.

Description
The Thundercloud is a bluish silver cloud with a smiley face and a yellow bolt of lightning above it, hence the name "thunder".