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Salamence's design is absolutely brilliant.
Bagon is not a flying creature, it does not develop rotating wings like traditional fliers. Through sheer willpower, it forced its body to adapt and mimic the shapes it tries to emulate, forming stiff, calcified 'wing' like structures from its cocoon as Shellgon. This is why they appear 'attached' to its natural body as opposed to extending from it.
Its 'wings' are only flexible enough to move slightly and do not create air vortices on their own. Rather, being a dragon, its body is full of gases used for breath attacks, which it uses to lift itself, hence its bloated-looking balloon shape. Then it uses its 'wings' for propulsion, handling and controlling drag. It also has a good feel for gravity and air currents from all its leaps of faith as Bagon, using them to aid in its flight.
Bagon hurts itself endlessly trying to fly, developing its cocoon as Shellgon to protect itself from impact. It then forces it to form a desired shape that allows it to fly despite its limitations, employing a unique method that uses its dragonic biology and aerodynamics.
Salamence is truly a masterwork of evolutoin.