Giant Flies are mutated insects featured in RAGE first appearing in the QuakeCon 2007 trailer. They can be seen in different places in-game, for example at Larkin's Meat Shop window in Wellspring.
Only somewhat bigger than ordinary pre-impact flies, they are seen clearly from a rather long distance when out in the Wasteland. They tend to stay at one place, producing a louder than usual sounds when Nicholas Raine steps into their territory.
These harmless creatures should be considered useful, as they appear near plant beds, dead bodies, wreckage or piles of junk where it's sometimes possible to find accidental loot. They have no other purpose in-game.