The Divine Condor had the pick of anyone to become the heir of his former master, Dook Goo Kau Bai, but for reasons unknown, chose Yeung Gor. Sure, Yeung Gor was himself a martial arts prodigy, but others living within the same timeframe also fit that description.
Dook Goo Kau Bai never appeared in the flesh in any of Jin Yong's stories, so we have no idea what he might have looked like. How's this for a theory: during his lifetime, DGKB greatly resembled Yeung Gor in his general look and bearing. Perhaps when the Divine Condor saw YG, he saw something of DGKB in the young man...something that reminded the creature of his departed master?