Either they were originally dragons in the first place — like Onox from the Legend of Zelda: Oracle of Seasons — or they became dragons by swallowing a Zonai Stone, like the Light Dragon and ...

Kotake and Koume also appear in The Legend of Zelda: Oracle of Ages & Oracle of Seasons. They send Veran and General Onox to bring about enough chaos to light the Flames of Sorrow and Destruction.