Nightmares

Nightmares are creatures that are created from bad dreams. It follows this system: The amount of bad dreams people have reflects the amount of nightmare creatures there are in Imagi-Nation.
 * Example: A person dreams about their teeth falling out, a creature that spits out his sharp teeth is created.

These creatures were created are an accumulation of all nightmares. They feed their own purpose and hurt others as they please.

As soon as the creator dies, the nightmare dies. That is why nightmares only have a certain life time-span.

Only those created by a dreamcatcher are strong enough to survive after the creator's death (hasn't happened yet.)