A Fly Guy is a type of Shy Guy that can fly using a propeller sticking out of its head. They first appeared in Super Mario World 2: Yoshi's Island. They are commonly found carrying items like Red Coins and even bombs. An egg can easily take it out. If eaten and spat back out it will become a regular Shy Guy. Another type of Fly Guy is the Leaf Guys, which are basically Fly Guys with leaves as their propellers. This variety appears only in Mario and Luigi: Bowser's Inside Story.
In the Mario Party series
Whenever a character used a Duel Capsule or initiate a duel on the board, a Fly Guy would come down to carry them off to the mini-game.
Fly Guys also appear in a handful of mini-games as well. In Mecha-Marathon, the characters must twist a wind-up key in order to initiate their mechanical Fly Guy in the race.
In the Super Smash Bros. series
In the Super Smash Bros. series, the Fly Guy appears in the stage Yoshi's Island seen carrying food, if you hit the Fly Guys, they will drop the food and fly away.
Fly Guys exist unused in New Super Mario Bros. Wii they're removed from the final game for unknown reasons they're scrapped enemies in the game