In Rayman and its spin-offs
In the original Rayman game, they are found in all worlds, except for the Dream Forest. There are many kinds of clouds. Some of them are static, and others follow various trajectories. Among static clouds, there are bouncy and trampoline clouds, the latter allowing Rayman to jump higher with the correct timing. There are also vanishing clouds, which sport a pair of eyes and disappear once Rayman steps on them, or at regular intervals. During the battle against Mr Stone, moving vanishing clouds appear, but these occur nowhere else.
In the Atari Jaguar version of the original Rayman, vanishing clouds open their eyes to reveal a malevolent face for a very brief moment every time they disappear. In any other version of the game, their eyes remain permanently closed but are still visible to the player, allowing them to quickly identify vanishing clouds.
In Rayman 3
In the Nintendo DS version of Rayman Raving Rabbids, clouds appear in several stages. They start out transparent and not solid, though they can be made solid by using the wind costume. They act as platforms used to get across large gaps. They will, however, only remain solid a set amount of time.