If you shake this egg, it'll cause the weather to change to night and rain for exactly 9.7 minutes within a 1 mile radius of where you shook the egg. So don't be worried if you see darkness in a small area. It, thankfully, is not a sign of impending doom.
It is rumored that Clous are the beings that control the weather. When they are happy, it's sunny outside. When they are mad, the weather becomes violent. But these are merely tales of old. It is likely that we will never know if they control the weather, because they cannot speak any human language (they speak Tenalp, a form of outer space communication).
Naturally, Clous are very bright and light up the night sky whenever you see them flying. They are overall happy creatures; they leave a trail of rainbow behind them that quickly fades.