Squidface the Terrible is mentioned in How to Train Your Dragon though he isn't actually seen. In How to Break a Dragon's Heart, he is revealed to be Stoick the Vast's father. He commented on Professor Yobbish's How to Train Your Dragon is comment can be seen on the back of the book.
Later on, when Hiccup asks Stoick: "What kind of father thinks his stupid Laws are more important than his own son?"
Stoick is seen saying: "It is WEAK of me, WEAK. Squidface the Terrible would have banished his son in the twinkling of an eye."
This suggests he was a ruthless, famous Viking.