A rare mob or monster is on a different timer than normal mobs in the area or sometimes only spawns X out of every Y instances you enter. Like elite mobs, rare mobs have a dragon around their portrait; however, unlike elites, this dragon icon is silver and lacks a wing on the side. Rare mobs also have a unique name that distinguishes them from normal mobs. They drop better than average loot, sometimes including loot specific to the unique mob. Their loot drops are almost exclusively within the uncommon table and below it (Low level rare mobs will often drop common loot.) Rare mobs found in instances may also drop an item from the rare table, which will frequently bind on pickup. Rare mobs found in Outland will only drop rare loot, which are all bind on equip.
Rares are sometimes erroneously referred to as "silver elites." However, unlike elite mobs (which have a gold dragon around their portrait), rare mobs are generally not tougher than normal mobs of the same level. Rare mobs typically have comparable health and DPS of a mob their level, but tend to have an extra skill or two. This skill can often be guessed by their name. There are also some mobs that are both rare and elite. Previously, such mobs had a portrait identical to that of an elite mob (gold dragon with a wing), making it indistinguishable without consulting some outside source. As of patch 2.0.1, elite rare mobs have silver dragon, like regular rare mobs, but with a wing, like elite mobs.
Rare mobs in the main game world have respawn times that are generally at longer intervals than normal mobs. Although it is difficult to determine the exact respawn rate of any given mob, some rares may take as many as several hours between spawns. As a result, some rares that are actively hunted by players may be very rarely seen.
Rare mobs in instancesEdit
Rare mobs in instances have a random chance of being generated each time you reset/enter the instance.
In actuality, there are several instances with mobs who are displayed with a silver dragon around it, yet spawn every time the instance is entered. This is because a patch changed them from a rare spawning mob into a normal spawning mob. Blackrock Depths has a number of examples of this, such as Pyromancer Loregrain.