WoW, had this great thing, which was called silver elites.
Silver elites would be rare spawnable bosses which had items which could not be obtained unless you killed that very mob, and that mob walked around carelessly and had a special name and looked slightly different from the others of that type of monsters.
Examples of how wow had it:
They would have a zone filled with raptors and one of them would be black but hard to find cause it moved all the time.
They would have a zone with wolfs, and one of them was a ghost wolf hidden in the mountains.
They would have a zone with humanoid mobs standing still and at the end a "Commander" which rarely spawned.
this was kinda fun, cause i just loved being a hunter and finding a rarespawn animal and capturing it as a pet which somewhat looked different with glowing eyes or something.