While transformation as the franchise knows it wasn't established until the Namek arc, the act of transforming has always been one of Dragon Ball’s gimmicks. The character Oolong is designed entirely around the concept, and the very first story arc ends with Goku undergoing a monstrous transformation at the sight of the full moon.
Source：https://www.cbr.com/dragon-ball-goku-transformations-how-he-got-them/
