S6, E8: Dean and Sam investigate what seems to be a werewolf killing but turns out to be a skinwalker (guest star ANDREW ROTHENBERG) posing as a family dog who is taking revenge on his owner's enemies. After capturing the dog, he turns human and tells Sam and Dean that an Alpha skinwalker has created many sleeper cells that will rise on his command and kill their families. The only way to stop the mass murders is to find and kill the Alpha.