It looks like a wind/debris deflector/mud flap/guard-type piece of some sort (please pardon my expert description). It looks like the main purpose of it might be to deflect debris from hitting the rubber boot around your axle (cv joint under it?). You could always try to check a junk yard and see if they have one that's in good shape or just take it to a dealer and show them and they'll charge you a boat load for a new one. Sorry, I don't know what it's called though.
07 WRX (Albins 5MT + Blouch 20G)
Jorge Carrillo Tuned (365 whp/340 wtq)