When a Bagon jumps off a cliff, it gets wounded and hardens its body. When a Shelgon jumps off a cliff, it shatters whatever it lands on, be it rocks, castles, or skyscrapers, or even against solid ground it leaves a crack in the earth. A Shelgon's wings are still generating inside their cocoon, but in a way they have learned to fly, if only for short distances; their hard body makes them the perfect live wrecking balls to suspend from a chain, or in days of old, live ammo for catapults.
Shelgon are both living and in possession of limbs large enough to let them maneuver, but small enough to remain aerodynamic. Because of this fact they can correct for the frequent inaccuracies, especially in older ages, inherent in many mechanical systems of propulsion. They are armored enough to smash into anything with little pain, and experienced enough from their cliff-jumping as Bagon to ignore what pain they do feel. This made them the perfect reusable ammo in the age of warriors, for if a castle was not breached Shelgon could simply run back home. If the wall was successfully broken, the Shelgon could then join in the fight, creating havoc where they ran and breaking shields as easily as walls.
From time to time a Shelgon evolved in the midst of battle. Most of these pokemon, volunteering for the army solely to experience the thrill of flight, spread their own wings and flew away from the war. But from time to time a Shelgon had grown loyal to its comrades, and when one did it meant victory for its side. The devastation inflicted by a single Salamence upon a battlefield is the primary reason knights so prized anyone brave and skilled enough to slay a dragon.