The Burden Beast, appeared in the episode "The Tale of the Forever Game", he was portrayed by Carl Crevier.
The Burden Beast, was linked to evil magical board game "The Forever Game".When a player summoned it to the woods, he would stick around and
try and hunt down anyone trapped in the woods. His reasons for hunting people is still unknown.
He has super fast speed, agility, stamina, and strength. Also he has long razor sharp claws, which he can throw like lawn darts.
When Nathaniel and Peter were playing the Forever Game, Nathaniel unleashed it into the woods outside.
That's where Peter's sister Monica and his friend Mark were trapped. He was out for the remainder of Peter and Nathaniel's game.
He wasted no urge and began to try and hunt down Monica and Mark, chasing after them and trying to throw his claws at them. They got away from him one time, when Monica left him a pouch of food distracting him. Also they hid behind the trees when it was dark outside.
He was last seen closing in on Mark and Monica, just as they were pushing down the weird tree over and the lightning came down, this possibly killed him. Also since the Forever Game was destroyed he is most likely destroyed now too.