Odin is the Norse ruler of gods and Asgard. He is mentioned many times, but never makes an appearance in the books or movie.
"Odin, it was rough, I almost gave up on you, and all the while you were holding out on me..." - Stoick, How to Train Your Dragon film
"By Odin's beard, Gobber..." - Stoick, Exclusive short Gift of the Night Fury
• It is known that Odin could possibly have a beard because of the phrase "By Odin's beard."