The danger dimension is known for bosses. To get to this dimension, look for an unstable ant and click it with an empty hand.  Multiple sets of ultimate gear and Big Bertha are recommended.  To get back home, just click another unstable ant.

Hostile Mobs: Terrible Terror, Creeping Horror, Lurking Terror, Cloud Shark.

Passive Mobs: Dragon, Gold Fish.

Bosses: Nightmare, Triffid.  From mob spawners: Nightmare, Jumpy Bug, Giant Worm, T-rex, Nastysarus, Mothra, Brutalfly, Mantis, Vortex, Emperor Scorpion, Caterkiller, Basilisk, Hercules Beetle.

Dungeons:  Mini Dungeons, The Challenge Dungeons, Cloud Shark Dungeon.