The so-called Tibia "Bosses" are unique in their own races. They are stronger than any of the creatures like them, and they are relatively rare to encounter.
Bosses do not spawn like normal creatures, they are only found in a world on rare occasions depending upon the abilities of the players in that world. If they are not killed in one day, then they disappear until another time. The most powerful bosses (like Orshabaal) are usually forewarned by multiple white text messages appearing onscreen, like the monster raids. Most other bosses randomly spawn without warning, providing an unpleasant surprise for anyone in the area.
Bosses are not restricted by the normal "anti-luring" boundaries, and they will not "poof" when they wander too far from their spawn point. However, some bosses disappear if they aren't killed in a certain amount of time (e.g. Ghazbaran) or if they have no target for a while.