From the Super Mario Wiki
“Hi! I'm Cyrrus! I'm a newborn cloud! Pleased to meetcha! I've got big dreams! Big ones! I wanna be the biggest cloud I can be! I heard about this one guy who could grow big just by eating a Mushroom. That would be SO COOL! If you could find a big-making thing like that, give it to me!”
Cyrrus is a newborn cloud in Super Paper Mario in The Overthere. His name comes from the given name Cyrus and from a type of long, thin cloud known as the "cirrus" (even though he more closely resembles a stratocumulus cloud). Tippi even mistakenly refers to him as "Cirrus" when tattling him right after meeting him.
If one of the four heroes brings him a Red Apple, he will grow large enough so that any character that jumps on him will bounce all the way into orbit before falling back down to a cloud above him.