Speedy Spirits are a type of ghost found in the game Luigi's Mansion, who, as their name implies, are speedy. Fifteen are in the game, and three of them can only be caught during a blackout phase in the game.
Speedy Spirits resemble the same figure as a Gold Ghost, only that they are a light blue color, not golden, and have 10 HP.
Speedy spirits are found in certain furniture of the mansion, and when said furniture is examined, the Speedy Spirit will appear, and if not immediately stunned by the flashlight, will dash around the room, never doing any harm to Luigi. Speedy spirits are not required to be caught to clear the room, but if Luigi wants a high score, he will probably want to catch all of them.
The main reason catching Speedy Spirits is beneficial is because of what they drop. Upon defeating a Speedy Spirit, they will drop a ton of money, and usually a gem (All of them drop Silver Diamonds in the PAL Hidden Mansion). However, there is a downside. If Luigi uncovers a Speedy Spirit, and he fails to catch it, it is gone forever, and the only way to get another shot at catching one is to reset the game, and go back to that room and try again.