They do, it's called a vulture, it's not in any of the Halo trilogy games nor mentioned in any of the books, but if you play Halo Wars, it's there. But Pelicans are also considered as gunships as their secondary role. Another possible answer to this question is the Shortsword, a lighter, faster version of the Longsword, but Shortswords are usually classified as bombers. Also there's the Hawk, or Sparrowhawk, that has a front-mounted Weapon/Anti-vehicle Model 6 Grindell/Galilean Nonlinear Rifle, or Spartan Laser.