According to the strategy guide by Alex Erins
"As bad as Necros are, they pale in comparison to Hellwings. Hellwings are basically baby dragons. They are hard to shoot, and they fire balls of flame at Duke from all directions. They will also knock Duke down (and possibly into water, lava, or off a ledge) at close range. Take these beasts seriously and try not to share a room with them. Use your RPG to clear the room, or your Energy Weapon to fry them in flight. Be sure auto-aim is on when you face Hellwings-drawing a bead on them manually is next to impossible."
They are encountered in old western times, medieval times and roman times.
Game manual description Edit
"Bat-like predators, these large flapping monstrosities belch sulfurous fireballs at any intruders into their territory. The Aliens use them as watchdogs to warn them of invaders."