Rudolph the Red-Nosed Reindeer/WMG
Rudolph is a Mutant.
He has the X-Factor in his genes, hence being able to emit a glow that can pierce through blizzards yet not blind people.
Hermey the Misfit Elf is actually a human being (albeit a short and/or young one).
Hermey differs substantially from the other elves --he has larger eyes, more hair, a nose that doesn't look like flesh-colored light bulb, and--most notably--rounded ears like those of a human. Normal elves may have an inborn desire to make toys (it's a desire that can be either genetic or passed on by magic,) but Hermey has no such inborn desire, choosing instead an alternative occupation like dentistry. (He also seems to have no interest in other elf activities like wiggling his ears, singing in a choir, etc.) This could be due primarily to his human heritage.
How did Hermey come to live among the elves? Perhaps he was an orphan who was accidentally taken by Santa (like in the movie Elf.) Or maybe he was left on the Claus' doorstep. The special doesn't seem to have much interest in his origin story, but a human origin would explain a lot of Hermey's differences.