The X-fighter was a type of Imperial ship.[1]
Behind the scenes
Appearing only in the new timeline introduced by the non-canon LEGO Star Wars: Rebuild the Galaxy, the X-fighter has a round cockpit section like a canon TIE fighter's and wings like a canon X-wing's.[2]
Appearances
LEGO STAR WARS: Celebrate the Season — "X-Wing vs TIE Fighter" (First appearance)
LEGO Star Wars: Rebuild the Galaxy — "Part One"
LEGO Star Wars: Rebuild the Galaxy — "Part Two"
LEGO Star Wars: Rebuild the Galaxy — "Part Four"
Sources
Get Ready for Darth Jar Jar, TIE-Wings, and More in LEGO Star Wars: Rebuild the Galaxy on StarWars.com (backup link)
LEGO Star Wars (Set: 75393 TIE Fighter & X-Wing Mash-up)