Björn Boar is a boar that Ruffnut and Tuffnut seem to know. He is seen once when attacked by a pack of Changewings and possibly saved. Björn looks just like an ordinary male boar. It is possible he lives on the edge because Ruff stated that Hiccup had to chase Bjorn Boar out of his house.
When the twins have a conversation:
Tuff:"Hey, don't we know that boar?"
Tuff "You leave Bjorn alone! "You're on your own, Bjorn!"
And they finally grabbed the Changewing's attention to them, even do it looks like they know the boar this could have just been one of their silly jokes.
The Changewings actually leave him and start to attack the dragon riders.