These are a higher level/stronger mobs in a particular region then the regular ones.
When defeating them you get a higher level of dropitems.
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 Bounty Monster will always stay at the same spot.
When you are 5 levels higher or lower, they will leave you alone.
A summary of all the Bounty Monster in the game: Category:Bounty Monsters