The Tantabus
Tantabus is a mystical entity from Equestria which Princess Luna created as personal punishment for her actions as Nightmare Moon. It is a shapeshifting blue mist which feeds on nightmares and grows stronger while doing so, and if powerful enough, could be able to escape into the real world where it can wreak nightmare havoc with no limits. It is currently imprisoned inside the subconscious of Luna right now. However, Luna doesn't realize that Tantabus wasn't actually created by her. It was yet another concoction of Pitch Black as a backup plan in case she should be reformed from her evil ways. He had cast a spell on her before she became Nightmare Moon which would make her feel so guilty, that she create Tantabus to punish herself. It is possible that this creature might make a comeback.