Copperhead Missiles were space cruise missiles employed by the Venomian Army during the Lylat Wars. Originating from the industrial planet of Macbeth, Copperheads were heavily ray-shielded and warp-capable ordinances that could heavily damage even large capital ships.
Copperheads were used during the Battle of Sector Z in an attempt to destroy Great Fox. Despite being escorted by the new Invader III Fighters, the Star Fox Team succeeded in destroying all six Copperheads used in the attack and save Great Fox from possible destruction.
During the Macbeth Campaign, one of the cars that Fox finds and destroys is a fuel car that is carrying a Copperhead Missile. As the Train was heading for a supply depot, this may imply that it was delivering the Missles to be launched.