A Mini-Boss is an enemy that can be found on the surface of any planet. Mini-bosses have more health than normal mobs, and are usually similar to other mobs on the planet in appearance and abilities, but are two times as large and have a red aura around them. The red aura may look like the particles emitted from a Red Stim Pack, but it is only a way of marking a mini-boss. They also appear to do more damage than their regular counterparts, but drop better loot; mini-bosses have a chance to drop legendary weapons, such as the Watersword or the Bone Hammer, when killed. Legendary weapons cast out certain, more powerful effects, such as burning, bolts of lightning, etc.
There can be an indefininte amound of mini-bosses on a single planet, but they are rare and finding multiple of them on one planet is very uncommon.
As of now, (V. Furious Koala) mini-bosses only spawn on the surface of a planet. They have no chance of spawning underground.