These are a higher level/stronger enemies in a particular region than the regular ones.
When defeating them you get a higher level of drop items.
Every 22 hours you can kill them again to get experience, but only the first time that you kill them you will get the maximum experience.
These Bounties will always stay at the same spot.
When you are 5 levels higher or lower, they will leave you alone.
The special event regions have also bounties: the event bounties.
A summary of all the Bounties in the game: Category:Bounties.
A summary of all the Event Bounties in the game: Category:Event Bounties.