Four winters ago I was listening to Carol of The Bells and started imagining seven people floating along the ground and up into the air. That lead to the idea of Santa's reindeer actually being humans who had the ability to fly and turn into reindeer.
I considered adding more reindeer markings on them, but decided against it. I sort of imagined there to be a whole story where it wasn't obvious that they were Santa's reindeer. Basically, it was supposed to be a story with Christmas elements, not a Christmas story.