A long time ago, in a land far, far away, Santa Claus was just starting his Christmas ministry. He was looking for animals that could fly, and pull his sleigh. Such animals are rare, indeed, because they have magical powers. Some animals, like birds, of course, fly under natural power, but unless you go back to primordial times, you won't find birds big enough to pull a sleigh loaded with toys.
So, Santa's list of potential applicants was very short. As everyone knows, there are elephants who can fly, like Dumbo, for instance. Unfortunately, they generally don't grow to full size, but remain very small, except, of course, for their ears. No, even a team of flying elephants could not pull Santa's sleigh.
Flying pigs are very rare, too. There's even a saying about that. Most of them come from Cincinnati, where, every year, they have a flying pig marathon. Flying pigs, the ones from Cincinnati, that is, have shown that they can go the distance. Unfortunately, they had a little problem with teamwork, and Santa quickly tired of trying to form them as such. Maybe someday, when the flying pigs from Cincinnati learn how to work together, rather than competing against each other, Santa can form them into a team to pull his sleigh.
Then there were the flying tigers, from whom the WWII fighter squadrons got their name, but these animals were a bit feisty, and they specialized in mortal combat. Santa couldn't persuade them to put their teeth and claws away, so that they wouldn't frighten the kiddies.
Contrary to rumor, there has only been one flying horse known in history, who went by the name of Pegasus. The history of Pegasus makes this very plain. Pegasus tried mating with mere terrestrial horses, but the act of mating got him so excited his wings would begin to flap and,... well, I'm sure you can figure out the rest.
As it happens, there have been two flying dogs known in history, one named Krypto and the other named Underdog. Either one could, theoretically, have pulled Santa's sleigh without assistance from anyone or anything else, but, alas, they were both busy with other pursuits. Besides, Santa had a lot of trouble communicating with Krypto (because he barked in code, don't you know) and he just wasn't sure how it would sound in song and story if it came to be known that his sleigh was pulled by an Underdog.
Well, to make a long story short, Santa tried several other flying animal teams, but after much research, he finally settled on a team of eight tiny reindeer. In spite of their diminutive size, they were powerful flyers and they worked very well as a team.
In fact, there was only one time when Santa had a problem with one of them, and that's because he was shy and standoffish, and very sensitive about his shiny nose that glowed in the dark. Fortunately for Rudolph (that was his name, you see) Santa knew just what was needed to persuade Rudolph to serve on his team. He temporarily furloughed his team of magic fireflies (the ones that powered his headlights) and gave the job of lighting his way to Rudolph.
[Hat tip to Dan Hampton for his plot idea. See "Guide the Sleigh: A Christmas Fable for Hamptons Kids".]