Flying Fish
Flying Fish are minor, tiny ghosts that only appear in the Courtyard and Pipe Room in Luigi's Mansion, and in the Ball Room in the PAL version of The Hidden Mansion. As the name suggests, the purple Flying Fish will pop up in groups and try to hit Luigi, causing five HP of damage for each hit. Like mice, they have no HP (as opposed to zero HP like Ceiling Surprise or Purple Bomber) and can be sucked up easily by the Poltergust 3000. The Flying Fish found in the Pipe Room are identical, except they are green and considerably faster than those found in the Courtyard.
Similar yellow ghosts appear in the Observatory, but they are bigger, shaped like comets, and can't be sucked up. They can, however, be aimed and fired at The Moon to create an ephemeral bridge leading to Mario's Star.