when did rudolph become a reindeer

Rudolph's story is a little different from the other reindeer. You see, he wasn't born a reindeer. He was actually a normal deer who lived in the forest with his mother. One day, he was caught in a snowstorm and became lost. He wandered for days, unable to find his way back home. He was so hungry and tired that he couldn't go on anymore. But then he saw a light in the distance. It was a group of reindeer, led by Santa himself!