In StarCraft, the Zerg flying unit that can morph into a Guardian is the Mutalisk.
The Mutalisk is a fast Zerg air unit used for harassment, scouting, and attacks against both ground and air targets. In the original game and Brood War, a Mutalisk can evolve into a Guardian after the appropriate technology is researched and the required morphing process is started.
Guardians specialize in long-range attacks against ground units and structures. Their transformation therefore changes a mobile, flexible aircraft into a slower siege organism. The two units are closely connected in the original Zerg tech tree, but they are not interchangeable: the Mutalisk is the starting unit, and the Guardian is the evolved form.
Scourge are another Zerg flying unit, but they function as expendable anti-air organisms and do not morph into Guardians. Devourers, introduced in Brood War, are also separate evolved Zerg air units.