The Reindeer Conundrum

In Clement Clarke Moore's classic Christmas poem "A Visit from St. Nicholas," Santa's seventh reindeer is referred to as "Donder."

But in "Rudolph the Red-Nosed Reindeer," the 1949 Johnny Marks song we've all grown up listening to, the same reindeer is called "Donner."

Is it "Donder" or "Donner"? What's the correct name of Santa's seventh reindeer? Answer here...