The Bolse Fighter is the name given to the starships that defend the Bolse Defense Satellite from would-be invaders. These highly-maneuverable ships were able to draw power from the satellite to power their own shield systems that proved nigh-impregnable to even the strongest fighter-grade weaponry.
The Star Fox team confronted these fighters when they launched their attack on the Bolse station. They were able to defeat the fighter's shields by knocking out their source of power, the force field that protected the station. With their primary defenses down, the fighters proved equally vulnerable to their weapons as any other Venomian craft.
Slippy Toad made a reference to the 1996 film Independence Day. When Slippy was trying to take down one of the Bolse Fighters, he says "Those ships are shielded too!", which is similar to what one of the character's lines in regards to not being able to penetrate the alien space-fighters shields ("Damn, they've got shields too!").