I'm sorry but H E A D C A N N O N okay so Dream being the human vessel for dream XD Nightmare and Lucid/dreamon dream dosent have control over them at most times so if he's angry sometimes Nightmare will come out and Nightmare being Nightmare would always be a pain to get back in to dreams like head so Nightmare can change his appearance to look like dream and Nightmare is the one who got dream in prison. That took to long